Challenges in Sustainability | 2014 | Volume 2 | Issue 1 | Pages 1‒17
Seeking Consilience for Sustainability Science: Physical
Sciences, Life Sciences, and the New Economics
Department of Community Development and Applied Economics, University of Vermont, 205 B Morrill Hall,
Burlington, VT 05405, USA; E-Mail: email@example.com; Tel.: +1 8026562989; Fax: +1 8026561423
Submitted: 29 January 2013 | In revised form: 10 April 2014 | Accepted: 11 April 2014 |
Published: 5 May 2014
Abstract: The human system, driven largely by economic decisions, has profoundly affected
planetary ecosystems as well as the energy supplies and natural resources essential to
economic production. The challenge of sustainability is to understand and manage the complex
interactions between human systems and the rest of nature. This conceptual article makes the
case that meeting this challenge requires consilience between the natural sciences, social
sciences and humanities, which is to say that their basic assumptions must be mutually
reinforcing and consistent. This article reviews the extent to which economics is pursuing
consilience with the sciences of human behavior, physics and ecology, and the impact full
consilience would have on the field. The science of human behavior would force economists to
redefine what is desirable, while physics and ecology redefine what is possible. The challenges
posed by ecological degradation can be modeled as prisoner's dilemmas, best solved through
cooperation, not competition. Fortunately, science reveals that humans may be among the most
cooperative of all species. While much of the mainstream economic theory that still dominates
academic and the policy discourse continues to ignore important findings from other sciences,
several sub-fields of economics have made impressive strides towards consilience in recent
decades, and these are likely to change mainstream theory eventually. The question is whether
these changes can proceed rapidly enough to solve the most serious problems we currently
Keywords: anthropocene; cooperation; human behavior; interdisciplinarity
Human impacts on the planet are now on the scale of
geological forces, to the extent that the current era is
increasingly referred to as the anthropocene .
These impacts threaten to exceed planetary bound-
aries, risking catastrophic impacts on humans and the
rest of nature . If we hope to meet fundamental
© 2014 by the authors; licensee Librello, Switzerland. This open access article was published
under a Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/).
human needs in the near term without destroying
planetary life support functions required by all species,
we can no longer separate the study of human systems
and natural systems, but must instead adopt a
transdisiplinary, holistic approach to science that "seeks
to understand the fundamental character of inter-
actions between nature and society. Such an under-
standing must encompass the interaction of global
processes with the ecological and social characteristics
of particular places and sectors." ( p. 641) Earth
Systems Science  and Sustainability Science 
exemplify this approach.
The steady accumulation of human knowledge has
made it impossible for any individual to be an expert in
all areas of study. Scientific progress therefore has
relied on increasing specialization in narrow areas, as
exemplified by the study of individual disciplines within
the universities. This specialization has resulted in
impressive advances, but it has also created barriers
between fields of knowledge. These barriers are not
only serious obstacles to the advance of sustainability
science, but also can lead individual disciplines to build
on beliefs or assumptions that contradict those of other
disciplines. Even if specialization is necessary, it is es-
sential that facts and theories within a discipline are
internally consistent, and the facts, theories and
inductions from one discipline do not fundamentally
contradict those from another. In particular, "when
different disciplines focus on the same object of know-
ledge, their models must be mutually reinforcing and
consistent where they overlap" ( p. 4). When there
is disagreement, it should be settled with empirical
tests, experiments and observations to lend support to
one hypothesis over another, and not simply ignored.
This is very much the case in the natural sciences.
Theories in cell biology do not contradict theories in
evolutionary biology, and both are consistent with the
theories of chemistry and physics. Even when facts and
theories may seem to contradict each other, as in the
case of quantum theory and the theory of relativity,
physicists pay close attention to the contradictions, and
assume that they will eventually discover basic physical
laws that resolve them. This type of agreement across
fields and disciplines is known as consilience .
Sustainability science demands consilience as an
Consilience is far less advanced in the social sciences
than the natural sciences . Facts, theories and
inductions from one social science not only frequently
contradict those from another, but also frequently
contradict the natural sciences. The most important
example of this in the context of sustainability science
may be in the discipline of economics for two main
reasons. First, economic activity—defined as the
transformation by humans of raw materials and energy
into goods and services intended to satisfy human
wants and needs—is the central cause of the most
serious sustainability challenges that human society
currently faces: global climate change, biodiversity loss,
land use change, ocean acidification, ozone depletion,
waste emissions in excess of the planet's absorption
capacity, and excessive dependence on rapidly dimin-
ishing stocks of fossil fuels. Second, economics
arguably has the most influence of any social science
on policy decisions. This article will focus on consilience
in mainstream economics, which has the greatest
impact on policy decisions, and hence the greatest
influence on sustainability.
Consilience is not the occasional incorporation of
theories or insights from the natural sciences into the
social sciences, but rather the explicit acknowl-
edgement that the social sciences must be consistent
with the common understanding of fundamental laws
that the natural sciences have built up over decades
and centuries. This does not mean however that the
social sciences should be explicitly modeled on the
natural sciences or should blindly adopt its methods.
There are profound differences between the two
fields. Theories in the social sciences can affect reality
while theories in the natural sciences cannot. For
example, if people believe the theory that abruptly
ending quantitative easing will cause the stock market
to crash, this could lead to a panicked sale of stocks,
triggering a crash. Eminent mathematical economist
Georgescu-Roegen argued that the mathematical
models of neoclassical economics—explicitly drawn
from the methods of mechanical physics—are ill-
suited for the modeling the qualitative change charac-
teristic of steadily evolving economies . Further-
more, though many physicist believe that if we knew
the position and velocity of all particles in the universe
it would be possible to retrodict the past and predict
the future, and some biologists believe that genes
determine behavior, the economy should not be
described as a mechanistic system devoid of purpose
and will, which leaves no room for policy . The
social sciences should be informed and shaped by, but
not reduced to, the natural sciences .
Consilience is also not a one-way street: economists
have long called for the natural sciences to become
more consilient with economics, complaining about the
arrogance of "some scientists in assuming that they are
competent to comment on the economic problems of
the environment without knowing any economics" .
Numerous economists have (correctly) pointed out that
limits-to-growth theorists since the time of Malthus
have often failed to account for role of the price
mechanism and human ingenuity in alleviating resource
constraints (e.g. [12‒14]).
Economics—conventionally defined as the allocation
of scarce resources among alternative competing ends
—is a broad field, characterized by many schools of
thought with different degrees of influence, some of
which have paid more attention to consilience than
others. Ecological and biophysical economics for ex-
ample explicitly strive for consilience with the natural
and social sciences [9, 15‒19], but these two fields are
rarely considered part of mainstream economics. It is in
fact a bit difficult to define mainstream economics
precisely. An entry in an on-line encyclopedia of
economics states that "we are all neoclassicals now…
what is taught to students, what is mainstream
economics, is neoclassical economics." . Precisely
defining neoclassical economics is also difficult. Some
authors identify three core axioms: economic phe-
nomena can only be explained as the result of
individual actions; all human behavior is an effort to
maximize the satisfaction of individual preferences; and
equilibrium between supply and demand is the starting
point for analysis [21, 22]. Other central themes found
in most undergraduate textbooks include the as-
sumptions that humans are rational, self-interested and
insatiable, everything can be measured in monetary
terms (monism), and preferences are exogenous;
furthermore, Knightian uncertainty (immeasurable risk)
is ignored, and the desirability of continuous economic
growth is taken for granted (e.g. [17, 23]).
In recent decades, serious theoretical and empirical
challenges to the core tenets of neoclassical economics
have shaken the field, and many economists argue that
mainstream economics is transitioning towards greater
consilience with the natural and social sciences.
Colander et al.  argue that at "the edge" of the
mainstream, leading economists are incorporating
complexity theory, psychology, ecology and institutions
into their theories. These leaders are strongly re-
spected by their more orthodox colleagues, resulting in
a continual evolution of the mainstream. However,
Colander et al. also acknowledge that the mainstream
economics of 15‒30 years ago (neoclassical economics)
is still taught to undergraduates. Hodgson speculates
that institutional and evolutionary economics may
become the new mainstream .
This article focuses primarily on the state of consil-
ience within mainstream economics, while acknowl-
edging the fuzzy boundaries of the field. At one
extreme the article will address the material taught in
undergraduate textbooks—hereafter referred to as
orthodox economics— which is the only exposure
most people receive to economics and arguably the
most influential on policy decisions . As Nobel
laureate and leading textbook author Paul Samuelson
stated "I don't care who writes a nation's laws—or
crafts its advanced treaties—if I can write its eco-
nomics textbooks" . At the other extreme this
article will address "the edge" of economics, and the
new ideas that may be filtering into the mainstream.
From the perspective of sustainability, however, The
most important area for consilience is in the advice
economists provide to policy makers.
Economics is conventionally defined as the al-
location of scarce resources among alternative
competing ends. From this definition, it follows that
two areas of consilience in economics are particularly
important. The first is the science of human behavior
(e.g. psychology, neuroscience, evolution, and so on),
which is relevant to both the ends that economic
activity should pursue and the institutions compatible
with human behavior. The second is the natural
sciences, particularly physics and ecology, which are
most relevant to understanding the means required to
achieve those ends. We can only decide how to
allocate resources after determining the appropriate
ends and human compatibility with different
institutional arrangements, and the available means,
including their physical characteristics.
The structure of the paper is as follows. The first
section following this introduction will focus on
consilience with the science of human behavior. Sub-
sections focus on rationality, self-interest and satia-
bility, followed by a discussion of the extent to which
consilience has occurred. The second section will
focus on the natural sciences, with sub-sections on
the laws of physics, and the laws of ecology, followed
by a discussion of consilience. The third section will
focus on the implications of consilience for the
allocation problem, with subsections on the physical
characteristics of the scarce resources, the laws of
economics, and how we should allocate.
2. Human Behavior, Ends and Institutions
Modern economics arose from utilitarian philosophy,
which viewed the maximization of utility—the achieve-
ment of the greatest happiness for the greatest
number—as a moral imperative for society and the
desired end of economic activity [26,27]. Since people
experience diminishing marginal utility, classical util-
itarian philosophy seemed to call for a more equitable
distribution of resources. Many economists argued
however that a major challenge to maximizing utility
was the difficulty or impossibility of objectively quan-
tifying utility or comparing utility between individuals.
On the other hand, if people are rational they will
prefer things that generate more utility to those that
generate less, and their willingness to pay for different
goods and services (including leisure and other non-
market activities, the costs of which can be inter-
preted as the income foregone by not working) will
reveal their preferences . There is no need to
directly measure utility. This result led mainstream
economics to redefine utility and welfare as the
satisfaction of individual preferences or tastes as
revealed by willingness to pay [26, 29, 30]. Utility for
society in the current period is therefore maximized
when resources are allocated to those willing to pay
the most for them, which also maximizes monetary
value for the economy as a whole. In the words of a
leading economist "the refusal of modern economists
to make "interpersonal comparisons of utility" means
in effect that they use wealth rather than happiness
as the criterion for an efficient allocation of resources"
( p. 88). By "efficient", Posner means Pareto
efficient (also known as Pareto optimal) in honor of
Vilfredo Pareto, a central figure in the development of
neoclassical economics. Pareto efficiency is defined as
a situation in which it is impossible to make at least
one individual better off without making another worse
off. Under certain rigid assumptions, markets can be
shown to allocate resources in a Pareto efficient
manner. The central desired end of economic activity in
mainstream welfare economics is Pareto efficiency,
equivalent to the maximization of monetary value or
economic surplus at any given point in time, brought
about by the satisfaction of individual preferences in a
market economy. Market competition however leads
firms to sell their products at the lowest possible price
to cover their factors of production, eliminating
economic profit. The pursuit of profit leads firms to
innovate, so that consumers can obtain more and
better products at a lower price over time. Since a
larger economy creates even more wealth, continuous
economic growth is another desired end.
These conclusions emerged from fairly rigid as-
sumptions about human behavior, including rationality,
self-interest, and insatiability. They also rely on the
assumption that we cannot make interpersonal
comparisons of utility. The following three sub-sections
will examine the rapidly accumulating evidence that
refutes these assumptions, much of which was
produced by economists at "the edge". The forth sub-
section below assesses the extent to which this
evidence has affected mainstream economics.
2.1. Are People Rational?
The behavioral sciences, including behavioral eco-
nomics and neuroscience, have done the most to
challenge the notion that humans are perfectly
rational. The polymath Herbert Simon first popularized
the ideas of "bounded rationality" and "satisficing",
which recognized that humans have limited cognitive
capacity and limited information, and under these
circumstances must settle for satisfactory rather than
optimal decisions . Tversky and Kahneman
showed in rigorous experiments that human decision-
making exhibits systematic biases. For example, most
people are risk averse, and weight losses more heavily
than gains of equal value. One can frame a single
problem in a way that emphasizes either losses or
gains and affect the decision making process . As
the title of two popular books emphasize, numerous
experiments have shown that people are "Predictably
Irrational" , and we can therefore use our know-
ledge of human behavior to "Nudge"  people in
desired directions. Economist Milton Friedman argued
that the test of a good theory is not its realism, but
rather its ability to generate good predictions ;
the assumption of economic rationality fails this test.
Neuroscientists and evolutionists have dug deeper
into the origins of such seemingly 'irrational' behavior.
The human brain has three quasi-independent
subsystems with different functions that evolved at
different times. Roughly speaking, the 'reptilian' part
of the brain is responsible for many automatic and
instinctual behaviors, the limbic system is responsible
for emotions and related behaviors, and the more
recently evolved neo-cortex is responsible for logic,
abstract thought and planning for the future. People
use different parts of the brain to make different types
of decisions, and it is possible to frame a decision in
such a way that it elicits a different response initiated
in a different part of the brain . Furthermore,
"continuous exposure to fixed cultural norms (e.g.,
religious doctrines, political ideologies and disciplinary
paradigms) literally helps to shape the brain's synaptic
circuitry in quasi-fixed patterns that reflect and embed
those experiences" which leads people to reject
information that does not conform to their pre-
existing beliefs . In fact, certainty appears to be
more of an emotional state than the result of rational
thought . This helps explain surveys that show
higher levels of education correlate with greater belief
in anthropogenic climate change in all groups in the
US except Republicans, where the inverse is true ;
and that conservative white males, particularly those
with high self-reported understanding of global
warming, are more likely to deny anthropogenic
climate change than other groups .
2.2. Are People Purely Self-Interested?
Convincing challenges to the notion of perfect self-
interest come from a wide variety of fields, including
anthropology, mathematical biology, behavioral eco-
nomics, neuroscience, epidemiology and evolution.
Increasing evidence suggests that symbiotic coop-
eration has played a critical role in major evolutionary
transitions, including the emergence of eukaryotes
from cooperating groups of prokaryotes and multi-
cellular life from cooperating groups of unicellular
organisms [42, 43]. Cooperation and concern for others
is ubiquitous in humans and likely the major factor
contributing to humanity's success [44‒47]. Historically,
economists used the theory of natural selection to
support their assumption of self-interest, arguing that
individuals who sacrificed their own fitness to help
others would be out-competed by selfish individuals,
thus purging altruism from the gene pool. However,
mathematical biology has confirmed at least five
different paths through which cooperation can evolve:
direct reciprocity, indirect reciprocity, kin-selection,
spatial proximity, and group selection. The fact that all
five occur in humans makes us 'Super Cooperators'
. Anthropologists have empirically tested various
theories of cooperation in modern communities, finding
significant support for them , while evolutionists
have tested their theories of cooperation against
random samples from the anthropological literature,
again finding significant support .
Perhaps the most interesting path to cooperation is
group selection, or more accurately multi-level se-
lection: groups with more altruistic individuals have
greater reproductive success than those with more
selfish individuals, even though within a given group,
selfish individuals may be more fit [44, 45, 49‒51].
This results in a population that can exhibit a wide
range of genetic pre-dispositions towards pro-social
behavior, ranging from purely selfish to highly altruistic.
Many cooperative species ranging from slime molds to
guppies and humans are able to detect and punish
cheaters and favor cooperators, which further pro-
motes cooperation [49, 52]. The need to identify
cheaters and cooperators may in fact have played an
important role in the evolution of human intelligence
In humans, the genetic capacity for cooperation
has been supplemented by culture in a co-evo-
lutionary process. Behavioral economists have devised
a series of games that show that people will sacrifice
their own welfare to help others even in anonymous
settings, and will also sacrifice their own welfare in
order to punish selfish individuals. Such punishment
appears to be a social mechanism for promoting
cooperation, and is thus known as altruistic pun-
ishment [54, 55]. Mathematical models show that
altruistic punishment, including the punishment of
non-punishers, greatly facilitates the emergence of
cooperation and is often built into cultural norms [48,
56]. As a result, different cultures exhibit different
degrees of cooperation [45, 57].
Another interesting finding is that cooperative species
as varied as the prokaryote
and the eukaryote
cooperate when resources are scarce, but not when they
are relatively abundant. This raises the interesting pos-
sibility that our competitive market economy is only
viable in the presence of fossil fuels, which unleashed a
new era of unprecedented resource abundance.
Confirming the biogenetic component of co-
operation, neuroscientists have drawn attention to the
neurotransmitter oxytocin and its kin, which are found
in all animals from fish to mammals. Oxytocin serves
as a hormone that stimulates birth contractions in
mothers, and as a neurotransmitter that induces a
strong feeling of bonding. When people engage in
cooperative activities, their oxytocin levels increase,
and administering aerosolized oxytocin increases the
likelihood of cooperation in experimental games [59,
60]. Oxytocin is also stimulated by sexual activity, and
induces sensations of well-being . Perhaps blood
oxytocin levels are a more accurate measure of utility
Humans are capable of developing institutions that
lead primarily selfish individuals to cooperate, or
primarily cooperative individuals to be selfish [22, 62‒
65]. One particularly disturbing finding is that monetary
exchange may actually reduce cooperation by crowding
out intrinsic motivations [66, 67], and simply priming
people to think about money may make them more
A final challenge to the assumption of perfect self-
interest is the compelling study by epidemiologists
Wilkinson and Pickett. Their research found that
individuals in unequal societies experience higher
levels of social and health problems then individuals in
more equal societies, regardless of overall levels of
income. In fact, wealthy individuals may be worse off
in unequal societies than lower income individuals in
more equal societies . Humans appear to have an
innate concern for fairness .
Integrating these insights into economic analysis has
profound impacts. Many of the most serious problems
faced by society today, ranging from climate change to
developing green technologies, can be modeled as
prisoners' dilemmas, which are best solved through
cooperation [45, 71]. If people evolved to be highly co-
operative, if different economic institutions elicit different
degrees of cooperation, and if markets can elicit selfish
behavior, then it becomes obvious that we must explore
a variety of allocative mechanisms in addition to markets
[22, 72], such as strategies based on shared production
and common ownership [71, 73‒77]. If people are
inherently social, then we must question the metho-
dological individualism that underlies most micro-
economic analysis. If people care about fairness and
equality, then just distribution may be more important
than Pareto efficiency.
2.3. Are People Insatiable?
The assumption of insatiability also fails to stand up to
the scientific evidence. Perhaps the most obvious
evidence comes from anthropology. Humans were
nomadic hunter-gatherers for at least 95% of their
history. As hunting and gathering activities depleted
food supplies, tribes were forced to seek out new food
sources, often far away. Those who attempted to
accumulate more than they could carry would starve
, so it is hard to envision an evolutionary advan-
tage to insatiability. Nomadic societies were also highly
egalitarian, and frequently punished individuals who
took too large a share of available resources .
Why then are people in general so willing to
consume more? Chilean economist Max-Neef suggests
that people might believe (perhaps convinced by
advertisers) that consuming a certain product will
satisfy their need for freedom, affection, participation,
leisure and so on. When consuming the product fails
to satisfy, they may mistakenly believe that they
simply have not consumed enough, leading to a
feeling of insatiability . Another problem arises
with positional goods, consumed to confer status.
Status is a relative concept, and if everyone's con-
sumption level increases equally, then status is un-
changed even as environmental impacts worsen .
Furthermore, as the rich increase their consumption of
status goods, everyone else will feel worse off, and
people may make important sacrifices of their own
well-being in other areas in order to maintain their
status [81‒84]. Humans are other-regarding in envy as
well as fairness.
This evidence suggests that beyond a certain point,
ever-increasing consumption, especially of positional
goods, may provide few benefits for society, while
imposing serious costs.
2.4. Is the Science of Human Behavior Affecting
Mainstream and Orthodox Economics?
Economists at the edge of the mainstream (including
many not cited above) have conducted much of the
research on human behavior that challenges rationality,
self-interest and satiability, with results frequently
published in mainstream journals and taught in
graduate programs. By these criteria, consilience is
However, while an increasing number of under-
graduate textbooks mention behavioral economics, it
has yet to change orthodox economics in any mean-
ingful way. Perhaps the most damning evidence here is
the consistently replicated research showing that
relative to the general population, people who study
economics on average behave less cooperatively [85,
86]; prioritize profit maximization over fairness ;
are more corrupt ; and are more likely to free ride
. Some of the differences are based on pre-
selection (i.e. more selfish people are likely to study
economics) but some are based on indoctrination ;
in either case this does not bode well for consilience
with the behavioral sciences. On the other hand,
Bowles'  textbook accepts the science of human be-
havior and evolution as core principles, and may fore-
shadow a fundamental change in orthodox economics.
From the perspective of sustainability, consilience
matters most when the resulting insights are
incorporated into policy recommendations. Gowdy and
Erickson (2005) argue that despite the fact that
"neoclassical theorists have by and large abandoned
economic man …the policy recommendations of
economists are still based on these outdated repre-
sentations of human behavior…(and continue) to offer
bad advice in dealing with some of the most pressing
environmental and social issues faced in the twenty-
first century". Gintis  concurs that "environmental
policies are generally based on a model of the human
actor taken from neoclassical economic theory".
Focusing specifically on the problems of positional
goods, but equally relevant to other insights from the
science of human behavior, Frank  asks "why does
the economics profession take no account of these
concerns when formulating economic policy
recommendations?", and asserts that none of the
responses provide by his colleagues bear scrutiny.
One reason that many economists fail to accept the
insights from behavioral economics is the assertion
that choice behavior is equivalent to welfare by
definition, in which case it simply does not matter
how or why people make particular choices. From this
libertarian perspective, if we cannot make objective,
interpersonal comparisons of utility, then the only
objective goal is free choice (e.g. [29, 30]).
However, when economists argue for the satis-
faction of subjective preferences as a central goal of
economics, they fail to point out that markets weight
preferences by purchasing power. Many of the prob-
lems central to sustainability concern the allocation of
society's shared inheritance from nature. It is hardly
value-neutral or objective to assert that we should
allocate based on the principle of one dollar, one vote
rather than one person, one vote, particularly if
people care about fairness. Markets assign a weight of
zero to the preferences of the destitute, and system-
atically allocate resources towards the wealthy. This is
particularly troubling for resources that are essential
and non-substitutable. Take food as an example.
When the prices of grain more than doubled during
the food crisis of 2007 to 2008, rich countries such as
the USA saw negligible change in consumption; the
price of wheat tripled, yet consumption actually
increased . The poorest countries in contrast saw
a dramatic surge in hunger and malnutrition .
Unquestionably, monetary value is maximized by
allocating food to an overfed rich person willing to pay
a high price rather than a malnourished and destitute
family who can afford to pay almost nothing, but it is
difficult to accept that this this is somehow optimal,
efficient or utility maximizing. In fact, markets
arguably allocate the marginal calorie to those who
gained the least additional utility from its consumption
. The refusal of orthodox economics to make
interpersonal comparisons of utility is so extreme
however that mainstream textbooks essentially deny
the distinction between wants and needs; to quote a
typical textbook, "the law of demand puts the concept
of basic human 'needs' to rest, at least as an
analytical concept" ( p. 259). Denying physio-
logical needs is denying basic science.
If I prefer oranges and you prefer apples, it may be
impossible to determine if the utility I receive from
oranges exceeds what you receive from apples, and
allocation based on willingness to pay seems perfectly
reasonable. However, to whom society decides to al-
locate resources required to satisfy physiological ne-
cessities is an ethical issue, not a question of pref-
erences [98, 99]. This is especially true if those
resources are a gift from nature.
While science can tell us much about the desirable
ends of economic activity, which ends should be prior-
itized is ultimately an ethical question. Science may
however be able to contribute insights into ethical
issues. One hypothesis is that ethics is the result of
gene-culture evolution designed to promote human
cooperation, and hence the survival of the species. Jot
down a list of five ethical behaviors and five unethical
behaviors. You are likely to find that ethical behaviors
put the group ahead of the individual, while unethical
behavior puts the individual ahead of the group. Most
religions similarly call for putting the group ahead of
the individual . Consilience with either the
sciences or humanities would force economists to
reconsider the goal of maximizing monetary value,
particularly for essential resources.
3. What Do the Physical and Life Sciences Tell
Us About Scarce Resources?
Consilience with the social sciences and humanities
would force mainstream economics to reconsider what
is desirable; consilience with the physical and life
sciences would force it to reconsider what is possible.
Conventional economics emerged at a time when
natural resources were vast relative to human
demands. The recently unleashed power of fossil fuels
provided access to previously unavailable mineral
resources and unprecedented quantities of renewable
resources. Surplus output allowed society to allocate
more resources towards science, and technological
advances further enhanced humanity's resource base
. Economists came to assume that technology
could always find a substitute for any given resource,
to the point that they could safely ignore natural
resources and focus entirely on capital and labor as
the scarce means of production . When
economists again began to occasionally incorporate
natural resources into their production functions in the
1970s, raw materials, capital and labor were treated
as substitutes, as though one could make more bread
from the same flour by adding more cooks and ovens
[102‒105]. Economists largely treated technology as
manna from heaven, and virtually ignored the
importance of cheap and abundant energy . The
power of fossil fuels however has allowed us to
deplete natural capital stocks and increase waste
emissions to levels that diminish the ecosystem's
capacity to reproduce and to sustain critical functions.
Economics can no longer ignore the laws of physics
and ecology and the natural resource base on which
society and the economy depend [9, 18, 107].
3.1. The Laws of Physics
From the laws of physics we know that it is impossible
to create something from nothing. All economic
products result from the transformation of raw
materials provided by nature. Furthermore, it is
impossible to create nothing from something. All
human made products break down, wear out and
eventually fall apart, returning to the environment as
waste. The extraction of raw materials from nature
and the return of disordered waste are known as
throughput. Simply maintaining existing capital stocks
in the face of entropy requires continuous flows of
We also know from physics that the transformation
of raw material inputs into economic products and
waste requires low entropy energy, irreversibly con-
verted through use into high entropy waste. Recycling
energy without net energy loss is impossible [18,
109]. Finite stocks of fossil fuels account for nearly
90% of all energy used for economic production. We
can use fossil fuels almost as fast as we like, but once
used they are gone forever. New technologies have
recently helped increase gross oil production, but with
increasingly high energy costs per new barrel ex-
tracted, hence lower net energy and higher green-
house gas emissions per barrel [110, 111]. The
renewable alternatives to fossil fuel are available in
vast quantities, but most are highly diffuse, difficult to
capture, transport and store, and flow at a fixed rate
over time [107, 112]. Sustainability demands that we
deplete fossil fuel stocks no faster than we master the
technologies required to bring alternative energy
sources on line .
Economists must account for at least three distinct
categories of factors of production, that are essentially
complements, not substitutes, and that have different
characteristics: raw materials, capital, and energy.
Raw materials—which Aristotle called material cause
and Georgescu-Roegen (1971) dubbed stock-flow
resources— are physically transformed into economic
products at a rate we choose, and use equals
depletion. Capital and labor—which Aristotle called
efficient cause and Georgescu-Roegen (1971) dubbed
fund-service resources—transform raw materials into
products that benefit humans at a given rate over
time. Fund-services are not used up in the act of
production, but rather are worn out and must be
maintained. Fund-services require energy, such as
fossil fuels, food or sunshine. As an example, a bakery
requires flour, cooks, ovens and energy (food for the
cooks, electricity for the ovens). Labor and capital,
both fund-services, can substitute for each other, but
are complements to flour and energy. A more efficient
stove uses less energy, which could be construed as
substitution at the margin, but once maximum
efficiency has been achieved, no additional sub-
stitution is possible.
Finally, the most basic laws of physics and
mathematics tell us that exponential increases in
efficiency or exponential growth of any physical
subsystem of a finite system can at best be transient
phenomena . One dollar invested in the year 0
at 6% interest would now have the same value as a
ball of gold (at $1300 an ounce) filling our solar
system to the orbit of Pluto. The economy is a
physical system, and cannot grow indefinitely.
3.2. The Laws of Ecology
The laws of ecology are almost certainly more tightly
binding on economic activity than the laws of physics,
though often far less understood. Many of the raw
materials (stock-flow resources) physically transformed
into economic products alternatively serve as the
structural building blocks of ecosystems (funds that
generate a service). Society can largely determine how
fast to deplete available raw material stocks, such as
trees in a forest. A particular configuration of eco-
system structure creates an ecosystem fund that
generates a flux of services over time. The ecosystem
fund is not physically transformed into the services it
provides (e.g. a forested watershed is not transformed
into flood regulation), but humans have little direct
control over the rate at which these services are
provided (a given hectare of forest can absorb only so
much water per day). When ecosystem structure is
removed and waste returned, often in novel forms to
which ecosystems have not had an opportunity to
adapt, ecosystem functions are affected: remove the
trees or kill them with acid rain, and rain water rapidly
flows over compacted soil into the adjacent river,
causing flooding downstream. Many of these services
are essential to sustaining life, including the capacity
for ecosystems to regenerate [114, 115].
Ecosystems exhibit the non-linearity, positive and
negative feedback loops, surprises and emergent
behavior characteristic of complex systems .
They are also poorly understood, so we rarely know in
advance the long (or even short) term impacts of our
activities [117, 118]. Many ecosystem services are
characterized by critical thresholds, beyond which
they will flip into entirely different states, potentially
far less amenable to the survival of humans and other
species, and this may hold true for the global
ecosystem as well. In most cases, we do not know
where thresholds lie, nor do we know precisely what
will happen if we exceed them [119, 120].
One of the major challenges in economics is to
determine how much ecosystem structure should be
converted into economic products, and how much left
intact to generate ecosystem services. Two basic laws
apply. First, humans cannot degrade or deplete any
element of ecosystem structure (e.g. fish, forests, or
fresh water) faster than it can regenerate without
eventually crossing some threshold beyond which that
component of the structure is gone, or else the
ecosystem itself crosses an irreversible threshold, with
often unpredictable but potentially catastrophic
results. Enough structure must be left intact to
maintain the flux of ecosystem services upon which
humans and other species depend. Second, humans
cannot emit waste into any finite system at rates
greater than it is absorbed, or else waste stocks will
accumulate, eventually harming humans and/or the
ecosystem in potentially catastrophic ways. Unfor-
tunately, failure to acknowledge the central impor-
tance of the life sciences to the field of economics has
led us to surpass these limits . It is now essential
to reduce resource extraction below regeneration
rates and waste emissions below absorption rates
until stocks are restored to levels compatible with
ecosystem resilience and the continued provision of
ecosystem services. The longer we take to accept
ecological limits to economic production, the greater
the reductions required.
3.3. Are the Sciences of Physics and Ecology Affecting
Mainstream and Orthodox Economics?
To achieve consilience with the physical and ecological
sciences, economists must place energy, natural
resources and waste at the core of their discipline,
and distinguish between fund-service (labor, capital,
ecosystems) and stock flow (raw materials) resources.
Economists must recognize that converting ecosystem
structure into economic products and emitting wastes
inevitably degrades ecosystem functions and accept
the urgent need to limit throughput to levels that do
not threaten life support functions of ecosystems. The
implications of these changes for sustainability are
obvious, but do not stop there. One reason that
economists pay little attention to distribution is that
their models show that factors of production (in-
cluding labor) are compensated according to their
marginal product, which is considered fair. Including
either natural resources or energy in economic
production functions reveals that factors of production
(e.g. labor and capital) are not awarded according to
their marginal product [122‒124], which would force
economists to pay more attention to distribution.
Acknowledging that virtually all economic activity
degrades ecosystem services inevitably leaving some
individuals worse off would force economists to
abandon the criterion of Pareto efficiency.
An increasing number of economists are acknowl-
edging that energy is an essential and non-sub-
stitutable factor of production [18, 123, 125‒127], but
many simultaneously acknowledge that "[v]irtually all
of modern economic growth theory assumes that GDP
growth per capita is driven by technological progress
and capital investment, including knowledge in-
vestment" and "does not take into account energy
availability or prices" . Similarly, many economists
recognize that nature provides essential and non-
substitutable benefits to humans while stressing that
mainstream economists assume endless substitutability
[19, 96, 128‒130]. The emerging field of degrowth
economics recognizes that the current level of eco-
nomic activity already overwhelms planetary bound-
aries and calls for economic contraction in the ag-
gregate to create ecological space for economic growth
in the poorest countries. Again however, these eco-
nomists almost inevitably distinguish themselves from
the mainstream, where the goal of endless growth is
considered the norm [131‒134].
In the second update to Barnett and Morse's 
Scarcity and Growth, Simpson et al.  provide an
excellent summary of neoclassical economists' treat-
ment of natural resources, energy and the environ-
ment as it has evolved over time. They conclude
(though do not necessarily agree) that "majority
opinion is that even in relatively short periods—years,
even months—substitution possibilities obviate re-
source scarcity" ( p. 6). The justification for this
belief is that scarcity leads to a price increase that
creates the incentives for substitution, efficiency, or
developing new substitutes. Orthodox economists often
view technology as manna from heaven, which allows
economic growth to continue forever. For example, a
classic article by Solow  shows technology con-
tinuously increasing the efficiency with which we use
fossil fuels so that we can continue to produce just as
much from ever smaller quantities. Economists at least
recognize that many ecosystem goods and services are
public goods that generate no price signal—a market
failure that must be corrected before substitution is
guaranteed. More modern growth models view tech-
nology as endogenous and also subject to numerous
market failures, but in general still conclude that
endless growth is possible as long as suitable policies
ensure adequate investments in technology .
Of course, few economists are calling for continuous
physical growth of the economy, but rather for pro-
ducing more from less, arguing that "nobody can define
a finite absolute minimum material input required to
produce a unit of economic welfare" ( p. 12).
However, just as one dollar grows exponentially in value
to equal a ball of gold the size of the solar system,
continuous exponential growth in economic welfare,
however measured, is likely to be equally impossible,
and would eventually lead to a state of relentless bliss.
Economics is a huge field, and undeniably a growing
number of economists are integrating ecology and
physics into their work, for example in the study of
ecosystem services, natural resources and climate
change [138‒140]. What really matters however is the
extent to which this translates into advice for policy
makers and education for the next generation of eco-
nomists. The standard proxy for the size of the eco-
nomy, GDP, makes no adjustments for the depletion of
raw materials or energy, yet most economists and
politicians alike call for its continuous growth, in spite
of its widely recognized flaws [141, 142]. College
textbooks in mainstream economics largely ignore
energy, and most focus entirely on labor and capital as
factors of production. Some textbooks do mention
natural resources, but invariably suggest that capital,
labor and technology are substitutes. Even more ad-
vanced courses in natural resource and environmental
economics generally assume unlimited substitutability
between raw materials and capital and focus on contin-
uous economic growth. Most mainstream economists
focus on Pareto efficiency, ignoring the fact that the
resource extraction, fossil fuel and waste emissions that
are the unavoidable consequences of economic activity
invariably have negative impacts on others.
4. Implications of Consilience for the Field of
If economics achieved consilience with other sciences,
it would be forced to completely rethink the problem
of allocation both within and between generations.
How we should allocate depends on the desired ends,
the physical characteristics and status (e.g. abun-
dance or scarcity) of the available resources, human
behavior and existing institutions. Some of these
factors are dynamic to at least some extent, and as
they change, so too must the institutions and mech-
anisms required for allocation.
4.1. Physical Characteristics of Available Resources
Before describing how true consilience would affect
economics, it is necessary to describe two important
characteristics of the available resources. The first is
known as excludability in economic jargon. A resource
is excludable if it is possible for one person or group
to use it while denying access to others. Access to
such resources can be rationed, which is necessary for
markets to exist. When a resource is non-excludable,
anyone who wants can use it, and rationing is im-
possible. Excludability is a policy variable that can be
implemented to different degrees, though some re-
sources, including many ecosystem services, are
inherently non-excludable. It would for example be
impossible to ration access to a stable climate or the
ozone layer's ability to protect us from ultraviolet
Rivalry is another important characteristic. A
resource is rival if use by one person leaves less for
others to use. All stock-flow resources are rival. For
example, if one person cuts down a tree to build a
house, that tree is no longer available for others to
use. Some fund-service resources are also rival. For
example, the more of the waste absorption capacity
for greenhouse gas emissions used by the USA, the
less is available for other nations. When global
emissions exceed absorption capacity, they accu-
mulate as harmful atmospheric stocks. A resource is
non-rival if use by one person does not leave less for
others to use. If a forested watershed prevents dam-
aging floods, landslides and erosion, the benefits
captured by one person living in the regions affected
do not leave less for others to use. Only fund-services
can be non-rival.
Non-rival resources are not scarce in an economic
sense and there is therefore no need to compete for
them. In economic terms, abundant, rival resources are
similar to non-rival resources. For example, a towel on
a beach or a car on the road leaves less space available
for another towel or another car, which is the definition
of rivalry. However, when available space on the beach
or road is abundant, there is no competition for use,
and the resource appears to be non-rival. Road tolls,
beach entrance fees or other policies can ensure that
spaces remain abundant. This has led many
economists to argue that rivalry is a policy variable. In
fact, rivalry is a physical characteristic that cannot be
affected by policy . No policy can change the fact
that burning a barrel of oil leaves less for others, or the
fact that additional people adopting a technology for
energy efficiency does not reduce its effectiveness for
those who adopted it first. Information is more than
just non-rival: it actually improves with use .
Information is also essential for all economic production
and must play an important role in addressing our
current ecological crises.
4.2. The 'Laws' of Economics
Modern economics emerged at the end of the 18th
century, when natural resources were relatively
abundant and per capita consumption of human made
goods and services a tiny fraction of what it is today
(Delong  estimates that real global GDP per
capita has increased more than 30 fold since 1800).
Increasing the output of human made products was
arguably the best way to improve human welfare, and
markets an effective way to achieve this goal. Most
economists therefore focused on market allocation.
The market price mechanism allocates scarce re-
sources towards the products that add the most
monetary value then rations those products towards
the consumers willing to pay the most for them. The
rule for achieving this outcome is to keep producing
or consuming until rising marginal costs equal
diminishing marginal benefits. The logic is straight-
forward: when marginal benefits exceed marginal costs
then increasing consumption of a single commodity or
of economic production as a whole increases total
utility. However, if marginal costs exceed marginal
benefits, then additional consumption makes us worse
off. Diminishing marginal utility, rising marginal costs,
and the equimarginal principle of optimization (i.e.
halting consumption when MB=MC) are treated as
basic laws of mainstream economics.
Mainstream economics generally focuses on
diminishing marginal utility for individual commodities:
the first 2000 calories you consume per day provide
far greater benefits than the next 2000. The same
rule applies however for aggregate consumption. In
general, people spend their first units of income on
basic necessities, such as food, water, shelter and
clothing. As we earn more income, we buy in-
creasingly less essential commodities with increasingly
smaller contributions to our well-being. This means
there are diminishing marginal benefits to increasing
consumption, and hence to economic growth. The
marginal costs of economic growth however are
rising. As individuals in a competitive market, we pay
the same nominal price for each additional unit we
consume. However, when we work to produce things
or earn money, we sacrifice the opportunity to engage
in other activities we might enjoy more. As we work
longer hours to earn more money, we must sacrifice
increasingly desirable alternative activities, so the real
cost of consumption rises. At the same time, if we
accept the laws of physics and ecology, for any given
technology, increasing economic production requires
the conversion of larger quantities of raw materials
and energy into economic products and waste,
sacrificing more ecosystem services. Logically, society
will sacrifice the least important ecosystems and
services first, and must therefore sacrifice increasingly
important services with increasing production.
Eventually, the rising ecological costs of economic
growth will exceed the diminishing marginal benefits,
and growth becomes uneconomic, meaning that it
makes us worse off . New technologies may
allow us to produce more from less, but there are
limits to efficiency improvements, and the inexorable
laws of exponential growth will ultimately take over.
Furthermore, efficiency improvements often result in
greater resource use, not less, and the same is true
for economic growth .
Eventually, as we convert more ecosystem
structure into economic products and return more
waste, we run the risk of crossing critical ecological
thresholds and imposing unacceptable ecological costs
on society. When we cross a threshold, a marginal
change in activity leads to a non-marginal change in
outcome. If the threshold involved leads to the loss of
an entire species or ecosystem, we must compare the
marginal benefits from the activity with the total value
of the species or ecosystem that is lost into the
indefinite future . Given the law of ecology that
everything is connected to everything else , the
total value may be unpredictable in advance, and may
not be realized for decades or even centuries .
Balancing marginal costs with marginal benefits is no
The central focus of an economics consilient with
laws of ecology and physics should no longer be
about maximizing the monetary value of market
goods and services. Rather, the first priority should be
to ensure that economic activity does not lead us to
cross critical ecological thresholds, ranging from
nitrogen emissions to biodiversity loss and climate
change. With current technologies, this may be very
difficult. For example, Rockstrom et al.  estimate
that nitrogen emissions must be reduced by 70% if
we are to avoid such thresholds, while greenhouse
gas emissions must be reduced by at least 80%
. With current technologies it is not obvious that
we can reduce emissions by that much and still feed 7
billion people. Major investments in research and
development in agriculture and clean energy will be
required. Even when safely distant from critical
thresholds, economists should focus on ensuring that
the marginal ecological costs of economic activity do
not exceed the marginal benefits, even though direct
comparison of the costs and benefits may be difficult
4.3. Resource Characteristics and Allocation
Markets are unlikely to be a suitable mechanism for
achieving these economic goals. In the absence of ex-
cludability, anyone who wants can consume a resource
whether or not they pay, and hence are unlikely to
voluntarily pay in a competitive market or a in a culture
that promotes self-interest. Ecosystem structure and
mineral resources are typically excludable under
existing institutions, and can easily be converted into
market commodities. However, the ecosystem services
lost from removing structure and emitting waste are
frequently non-excludable. Markets do not compensate
for their provision or penalize for their loss. The result
is over consumption, under-provision and degradation.
This dynamic explains anthropogenic climate change,
land use conversion, biodiversity loss, and most of the
other serious problems currently faced by society.
One solution is to make the resource excludable so
that it becomes possible to ration access. It is impos-
sible to make services such as climate regulation,
disturbance regulation or protection from UV radiation
excludable, but it is generally possible to regulate or
make excludable the activities that destroy these
services. However, making something excludable
requires collective action via social institutions; it is a
prerequisite for market allocation, and not the result of
markets. If sustainability is a goal, then society must
step in to regulate access to ecosystem structure and
waste absorption capacity to ensure the adequate
provision of ecosystem services. Mainstream eco-
nomists often argue that simply establishing tradable
private property rights will automatically lead to
efficient allocation, so who receives those rights is
relatively unimportant [151, 152]. However, as we saw
above with the case of food, market allocation often
forces those with the greatest level of physiological
need for a resource to reduce consumption the most. If
we limit land use change, biodiversity loss, freshwater
and nitrogen to ecologically sustainable levels, food
prices will likely skyrocket and the poor will starve,
which is not socially sustainable. If humans do indeed
care about fairness and the well-being of others, then
price-rationing of essential resources—especially those
freely provided by nature—is inappropriate. Delib-
erative democratic processes give equal weight to
everyone's preferences, while markets weight
preferences by purchasing power. Which of these
approaches to use is about the distribution of power.
Furthermore, ubiquitous externalities rule out Pareto
efficiency as a useful criterion, since virtually all
economic activities have negative impacts on others.
But rationing access is not always a solution. Non-
rival resources are not depleted through use, and
rationing access therefore reduces benefits without
affecting costs. Such resources are not scarce in an
economic sense, as there is no need to compete for
them once they exist—though there is competition for
any rival resources that might be required to produce
or protect them. Markets are only efficient (i.e. able to
balance marginal costs with marginal benefits) for
resources that are rival. Paradoxically, the economic
surplus (the monetary value of total benefits minus
total costs) from non-rival resources is maximized at a
price of zero where anyone who wants can consume
the resource. This is especially true for clean
technologies that replace polluting ones. However, at
a price of zero, market supply is also zero. Economic
systems must still allocate resources towards the
production or protection of non-rival resources.
Private property rights to non-rival resources, (e.g.
patents) provide market incentives to supply them,
but simultaneously reduce the economic surplus they
generate. The appropriate allocation mechanism is
some form of cooperative (e.g. publicly financed)
provision that rewards innovators while making their
innovations freely available [71, 153].
Many of society's most important resources,
ranging from global climate stability and clean energy
technologies (information) to biodiversity and critical
ecosystem services, are non-rival and inherently non-
scarce, challenging the very definition of economics.
Most of these resources are also inherently non-
excludable so that rationing access is also impossible.
However, global society has been strengthening
intellectual property rights for decades, using prices to
ration access to many of the technologies required to
solve our global problems . For example, if we
develop a clean, efficient, decentralized form of solar
energy, no matter how much solar energy one country
captures, there will be no less for others, and the
technology itself is likely to improve through use.
Patenting the technology and charging for it will
reduce use and hence the potential for reducing
climate change .
If people were inherently self-interested and
competitive, as typically modeled by orthodox eco-
nomists, then we would be forced to rely on economic
institutions that channel that behavior towards the
common good, such as markets. Behavioral sciences
however show that humans are capable of cooperation,
and can build institutions that enhance our innate
propensity for pro-social behavior. As discussed above,
markets may actually undermine cooperation.
If economists hope to contribute to sustainability
science, they must take a scientific approach to
economics that builds on insights from the physical,
life, and social sciences. Objective physical charac-
teristics of resources, not ideology, determine whether
competitive or cooperative allocation is most efficient.
Table 1 briefly describes potentially suitable mech-
anisms for allocating different types of resources.
While versions of this table are fairly standard in the
economic literature, most economists treat problems
resulting from non-excludability and non-rivalry as
market failures, externalities that should be inter-
nalized through market prices. An economics that was
more consilient with advances in other fields would
instead recognize that economic activity unavoidably
degrades the environment, environmental degradation
is one of the greatest threats to human welfare, and
most environmental problems take the form of
prisoners' dilemmas that can only be solved through
cooperation. Economics should therefore strive to
develop the cooperative institutions required to solve
these problems, and abandon its obsession with
private property and markets.
Table 1. Resource Characteristics and their Implications for Allocation.
Rival and scarce Potential market goods, e.g food, oil,
land, consumer goods, but with
inevitable negative externalities, ruling
out Pareto efficiency as a decision tool.
Rationing is desirable, but price
rationing of essential resources is
Open access regimes e.g. absorption
capacity for greenhouse gasses;
oceanic fisheries: rationing is
desirable, but requires cooperation and
(club or toll
Club or Toll goods e.g. beaches, parks:
rationing desirable when scarcity is a
Rationing is desirable when scarcity is
a threat, but requires cooperation and
Non-rival Tragedy of the non-commons e.g.
patented green technologies:
rationing undesirable. Open access is
more efficient, but requires
cooperation and collective action.
Public goods/open access resources
e.g. climate regulation, flood
regulation, open source information:
rationing undesirable and impossible.
Cooperative provision is essential.
There is little question that the discipline of economics is
in a rapid state of flux. Leading economists at the edge
of the mainstream are undoubtedly incorporating ideas
from the science of human behavior, physics and
ecology, and these are slowly filtering down into the
mainstream. Even ideas from decidedly non-mainstream
fields such as ecological and biophysical economics are
becoming more widely accepted. Consilience is oc-
curring. Unfortunately, there is less evidence that the
sciences are having much impact on the economic
orthodoxy, which is widely taught to undergraduates, or
on the advice given to policy makers. Perhaps this is to
be expected however, as academic disciplines tend to
At the same time however, the economic system is
also in an extremely rapid state of flux, and its impacts
on global ecosystems are unfolding at an unprecedented
pace. Since the 1950s, the human population has more
than doubled, the use of petroleum has nearly
quadrupled, and economic activity has increased by a
factor of fifteen. Ecological impacts have increased at
the same pace . Economists can no longer afford to
ignore basic principles of ecology and physics. Solving
these problems will require new economic institutions
based on cooperation, and such institutions must be
based on detailed knowledge of human behavior. A few
decades ago, stocks were long-term investments held
for years. Today, they are held for seconds [155, 156].
Foreign currency transactions used to be strictly
regulated. Today, annual transactions are more than
twenty times global GDP . In complex systems,
such rapid and powerful changes can have profound
impacts, such as the financial crisis of 2008, which
caught most economists completely unaware. The
financial crisis undoubtedly pales in comparison to the
more slowly unwinding ecological crises we now face.
Economists can no longer afford to ignore the fact that
the ecological-economic system is a complex, adaptive
system subject to surprise and emergent behavior.
Economic theory must evolve at least as fast as the
economic system if it is to help society face 21st century
challenges. When unfolding events falsify theories from
mainstream and orthodox economics, those theories
must be abandoned. We cannot passively await progress
at the edge of economics to filter through to
practitioners and textbooks over coming decades.
Consilience must be aggressively pursued as a core
principle of economic theory.
I would like to thank the Vermont Agricultural
Experiment Station Hatch Program for the funding
necessary to write this article. I also wish to
acknowledge the extremely useful comments made by
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