Philosophy of Chemistry�A New Interdisciplinary Field?

JChemEd.chem.wisc.edu • Vol. 77 No. XX Month 2000 • Journal of Chemical Education 1
What could possibly be the connection between chem-
istry and philosophy, apart from the obvious superficial one
of their both representing quests for knowledge? How do
contemporary chemists and philosophers generally view one
another? These are some of the questions I will try to put
before going on to describe the connections that have recently
been forged between these two seemingly very diverse fields
of academic study.
The View from Each Side
I think it is fair to say that chemists and philosophers
traditionally regard one another with a certain amount of
suspicion and in some cases maybe even a little contempt.
Chemists rightly feel proud of the fact that they engage the
phenomenal world through experimentation and are prepared
to revise their theories and practices accordingly. To the
chemist, the philosopher—who conducts no experiments
whatsoever—is not worthy of very high esteem. From the
scientific perspective, philosophical views do not seem very
dynamic, since they sometimes stem from established philo-
sophical doctrines or a priori beliefs about the ways the world
should be.
Philosophers for their part are proud of their training in
rigorous ways of thinking. They freely admit to not engaging
in the grubby details of the experimental world because such
activities might limit the generalities of their claims and their
attempts to depict reality in its broadest terms. Some of them
may even secretly look down at chemists for taking scientific
models literally, for being naive realists, and perhaps—to use
some well-known pejorative labels—for being “stamp collectors”
and “pot boilers”. Of course all these introductory remarks
are intended by way of caricature; but although it is simplistic
to accuse educated folk of such narrowmindedness, I believe
there is at least some truth in what I am suggesting.
Most philosophers of science believe that chemistry has
been reduced to physics and is therefore of no fundamental
interest. They believe that chemistry has no “big ideas” to com-
pare with quantum mechanics and relativity in physics and
Darwin’s theory in biology. Furthermore, given their relative
lack of interest in experiment, as opposed to theory, it is not
surprising that philosophers have tended to ignore that very
experimental science—chemistry.
New Thoughts on Chemistry
It may therefore come as something of a surprise to the
reader to learn that during the past ten years or so there has
been a veritable upsurge of interest in the field of philosophy of
chemistry. All the above misconceptions have begun to be
questioned and philosophers of science now realize that while
they have paid great attention to physics and more recently
biology, they have almost completely neglected the central
science of chemistry (1).
At the same time, several chemists and commentators
have produced deeply reflective books and articles in which
they explore the essential nature of chemistry and the
chemist’s practices and particular way of thinking (2). There
are now both an international society and two international
journals1 devoted to philosophical aspects of chemistry. In
addition, a lively Internet discussion list called “philchem”
has been in existence for about three years.2
The Question of “Reduction” in Chemistry
Among the works produced by philosophers of chemistry
is a critical analysis of the question of the reduction3 of chem-
istry to physics (3), something that seemed to have been
regarded as a foregone conclusion by the previous generation of
philosophers (4 ). The term reduction refers to the increas-
ingly prevalent view, which has had a large impact on chemical
education, that all the deep questions in science can be re-
solved by appealing to the more fundamental theories found
in physics. The notion that chemistry does indeed reduce to
physics is implicit in the increasing use of physical principles to
explain atomic structure and the periodic system, for example.
Similarly, many other areas of chemistry, including inorganic
chemistry, are approached through physical principles rather
than by focusing on qualitative aspects and the diversity of
observed phenomena. As educators at all levels are well aware,
part of the art of teaching chemistry lies in knowing how to
maintain a balance between these unifying principles and the
more apparently diverse qualitative and descriptive aspects.
This issue is also important to writers of textbooks and those
designing instructional educational software.
One aspect of recent work in philosophy of chemistry is a
willingness to explore the alleged reduction of chemistry to
physics by examining just how much can be derived from
ab initio quantum chemistry instead of the more traditional
philosophical approach, which involves the axiomatization
of theories and laws. The archetypal example of an axiomatic
system is Euclidean geometry, in which the entire system can
be represented as a set of precise and clearly stated rules. Any
competent user of these rules can prove other propositions
within the system with the utmost clarity and certainty. Similar
axiomatic approaches developed in physics may have some
value in establishing whether thermodynamics is reduced to
statistical mechanics, for example, since some reasonably
satisfactory formalized versions of these two branches of
physics are available. However, in the case of chemistry, at
least two problems render this approach to the question of
reduction highly problematic. First, it is by no means clear
that chemistry possesses any exact laws of the same form as
Newton’s laws of motion or other physical laws. Second, the
notion of axiomatizing chemistry appears to be rather ill
founded, given the relative lack of rigorous mathematical
Philosophy of Chemistry—A New Interdisciplinary Field?
Eric R. Scerri
Department of Chemistry, Purdue University, West Lafayette, IN 47907; scerri@purdue.edu

2 Journal of Chemical Education • Vol. 77 No. XX Month 2000 • JChemEd.chem.wisc.edu
theories in the chemical sciences (5). This is why philosophers
of chemistry have adopted the alternative approach mentioned
above and have examined the extent to which chemical data
can be predicted from first principles. One of them suggests
that some quantum chemists overstate their claims to having
reduced chemistry to quantum mechanics, even in the limited
domain of calculating energies or bond angles, and that
chemical educators need to take note of this situation (6 ).
But Chemistry Does Have Laws
I am not trying to imply that chemistry does not possess
any laws. However, the periodic law of the chemical ele-
ments, for example, differs from typical laws in physics in
that the recurrence of elements after certain intervals is only
approximate. In addition, the repeat period varies as one
progresses through the periodic system. These features do not
render the periodic law any less lawlike, but they do suggest
that the nature of laws may differ from one area of science
to another. Perhaps chemical laws should not be judged to
be deficient by the standards of laws of physics.
The periodic law and the associated periodic system
represent what I believe to be one of chemistry’s “big ideas”.
I do not need to stress to this readership the ubiquity of the
periodic system in teaching and chemical research, and the
historical role it has played in the foundation of atomic physics
and quantum mechanics through the contributions of
Thomson, Bohr, Pauli, and others (7 ).
Models and Explanations in Chemistry
Similarly, the nature of chemical models is providing a
rich source of examples to philosophers who are interested in
obtaining a wider view of these scientific entities. In addition,
chemical educators have been drawn into philosophy of
chemistry in an attempt to clarify the meaning of terms like
model and law as employed in teaching chemistry (8). This
kind of research is clearly needed in chemistry, which, to the
outsider, appears to be guilty at times of adopting conflicting
models to explain particular chemical facts. How many of us
have experienced students’ frustration when we give different
chemical explanations depending on the context in which one
and the same phenomenon is being discussed?
If one believes only in fundamental explanations, this
form of activity appears to be seriously mistaken. However,
as chemists we are also aware of the need to operate on many
levels and the fact that explanations can be genuinely level-
specific. Such approaches must be used very carefully. They
should not degenerate into the introduction of ad hoc explana-
tions that are invoked in the explanation of particular
chemical facts but cannot be generalized to other situations.
One example is the wide variety of explanations given for
the apparent orbital paradox concerning the relative occupation
and ionization of the 4s and 3d levels in the first transition
metal series. The paradox I allude to is that the 4s orbital
is preferentially occupied but also preferentially ionized.
Nobody has yet rationalized this situation at a level that might
be appropriate for teaching general chemistry. Most educators
and textbooks continue to argue that the 4s orbital is prefer-
entially occupied because it has a lower energy than 3d, in
spite of several articles published in this Journal that state
that the 4s orbital never has a lower energy than the 3d (9).
Another response, encountered particularly among theo-
reticians, is that this is a futile question because the concept
of orbitals ceases to refer to any objective entities in more
advanced calculations and can only be maintained at the level
of the Hartree–Fock approximation. I suggest that this kind
of response is just another way of expressing Dirac’s famous
dictum whereby chemistry has been explained in principle
by quantum mechanics. Such a response amounts to evading
the issue, which is to try to obtain a consistent explanation
within an orbital approximation such as the Hartree–Fock
model, since within this regime the concept of an orbital is
well defined.
Realism
Realism and Molecular Structure
The question of realism regarding scientific terms in
chemistry has also been revisited in recent articles in philoso-
phy of chemistry. For example, the commitment to realism,
which some believe to be a feature of chemistry, has received
a serious challenge in the form of the molecular structure
controversy that has been the feature of at least two articles
in this Journal (10).
Woolley and other authors suggest that the concept of
molecular structure, which is so central to modern chemistry,
is nothing but a metaphor having no objective reality at the
quantum mechanical level. The basis of this claim lies in the
fact that the appropriate Hamiltonian used in quantum
mechanical calculations for a molecule such as C3H4 only
contains terms describing interactions between protons and
electrons in the system. Woolley claims that the structure of
the molecule (or the relative positions of the nuclei) is in-
troduced somewhat artificially in calculations by invoking the
Born–Oppenheimer approximation, which assumes that only
the electrons move within a rigid framework defined by the
positions of the nuclei, which are assumed to be fixed in space.
This approximation is based on the large differences in mass
between electrons and nuclei, with the assumption that the
electrons can respond instantaneously to changes in position
of the nuclei.
Woolley and others have claimed that a purely quantum
mechanical description involving the raw molecular Hamil-
tonian without use of the Born–Oppenheimer approximation
does not require the attribution of any structure to molecules.
The sharing of the same Hamiltonian by the three known iso-
mers of C3H4 (see structures below) implies that a purely quan-
tum mechanical calculation without the Born–Oppenheimer
approximation cannot distinguish between these structures.
Nor, it is claimed, are such considerations purely academic;
as Woolley pointed out, in many cases calculations carried out
without the Born–Oppenheimer approximation yield more
accurate predictions than those carried out by the more con-
ventional approach, which does make use of the approximation.
CH2CH3C CH2CH2C C
H
C
C
H
H H
methyl acetylene allene cyclopropene
Most chemists react with complete incredulity to the
view that structure is nothing but a metaphor, pointing out the
seemingly overwhelming evidence for structure that comes

JChemEd.chem.wisc.edu • Vol. 77 No. XX Month 2000 • Journal of Chemical Education 3
from spectroscopic and other structural studies. They suggest
that if a deep quantum mechanical analysis reveals molecular
structure to be a mathematical artifact, then the fault must
lie with present-day quantum mechanics and not with the
deeply entrenched chemical notion of structure. Interest-
ingly, a philosopher of science has sided with the traditional
chemical view in upholding the reality of molecular structure.
Ramsey argued that careful analysis of the work of Woolley and
some other authors reveals that they are themselves misinter-
preting what it means to hold realistic views about scientific
entities, but this debate is far from being completed (11).
When Is Realism Appropriate?
I believe that chemists do in fact have a tendency to be
naive realists and that while this philosophical attitude is
perfectly appropriate in some instances it is not in others. It
is appropriate in the sense that chemists operate with what
have been termed secondary properties such as colors and
smells. For example, the color of chlorine is correctly regarded
in chemistry as simply “green”, without specifying the exact
frequency of its color. Properties such as color can be usefully
regarded as characteristic of the compounds themselves in much
chemical work. However, when dealing with properties that
originate in quantum mechanics, such as atomic orbitals,
should chemists defer to the physicist and accept what quan-
tum mechanics might have to say on the subject? This is what
I formerly recommended, but I no longer believe that matters
are so simple (12).
In modern chemistry such terms as caloric and phlogiston
have long been dismissed because they are thought to be non-
referring; that is, we believe that nothing exists in the world
to correspond to such terms, which were formerly invoked in
chemical explanations. Nevertheless, the term atomic orbital—
which is also strictly nonreferring, unless one is concerned
with the hydrogen atom—continues to be used in chemistry.
In fact, orbitals and the related concept of electronic con-
figurations, neither of which truly “exist” in many-electron
atoms according to a strict interpretation of quantum me-
chanics, have become the central paradigm at all levels of
chemistry.
Can Orbitals Be Real in Chemistry but Not in Physics?
This situation raises a philosophical question regarding
the status of orbitals and configurations. Although they may
not exist in the context of quantum mechanics, both concepts
serve as a very useful approximation, which clearly should
not be abandoned. This issue is complicated by the fact that
computational chemists use orbitals and configurations as
mathematical fictions, whereas chemical educators tend to
attribute something of a definite existence to them. Perhaps the
current emphasis on teaching chemistry in a quasi-deductive
manner, starting from the configurations of the elements, does
chemistry a disservice by making an approximate model appear
to be more concrete than it deserves. This situation is exac-
erbated by the tendency of some chemistry instructors and
textbook authors to teach electronic configurations by means
of drill exercises, thus further incorrectly suggesting the fun-
damental nature of orbitals and configurations. One rather
extreme example of this tendency for drilling configurations
into students can be seen in the otherwise excellent World of
Chemistry video series.
If one takes a fundamentalist approach, it emerges that
quantum mechanics denies the very existence of orbitals and
configurations in many-electron atoms, rather than under-
writing them as is generally supposed. However, chemistry
is not physics, and it may well be that an approximate concept
is very useful in chemistry despite being deemed nonexistent
by the more fundamental discipline of physics. The way out of
this impasse might be to uphold the autonomy of chemistry
and to use what might be termed “chemists’ orbitals”. These
entities would correspond to the electron orbits discussed just
prior to the discovery of quantum mechanics, when orbits could
still be regarded as “real” entities. In the case of these chemists’
orbits one would need to retain most of the results from
quantum mechanics, such as the probabilistic interpretation, but
one might want to ignore the modern quantum mechanical
finding that the assignment of four quantum numbers to each
electron is strictly invalid.
Such a view of autonomous levels of science, which is
gaining prominence in philosophy of science, would imply
that chemistry can make use of fundamental physics by
borrowing such terms as orbits and configurations but would
not be required to follow strict physical laws to the point of
denying the existence of atomic orbitals in chemistry (13).
Where Is Naive Realism Inappropriate?
Fritz Paneth, one of the founders of radiochemistry and a
highly reflective scientist, once suggested that although
chemists have much to gain from adopting a naively realistic
attitude in refusing to follow the latest discoveries from physics
at every turn, there is also a price to be paid. According to
Paneth, chemists must abandon naive realism if they are to
make any sense of the notion that elements persist in com-
pounds they might form. For example, sodium as the gray
metallic substance is nowhere to be found in the compound so-
dium chloride. Paneth’s suggestion for overcoming this prob-
lem is to argue for a metaphysical or transcendental view of
the elements as abstract bearers of properties that are them-
selves unobservable. As Paneth reminds us, it was this under-
standing of the term element, sometimes referred to as “basic
substance” rather than the directly observable or “simple
substance”, that Mendeleev adhered to when he constructed
his periodic system.
Paneth also suggested that it was because Mendeleev held
this kind of metaphysical view of the elements that he believed
so strongly in the periodic law, even in instances where experi-
mental evidence appeared to contradict it (14 ). Perhaps it
is not that Mendeleev’s contemporaries just lacked the cour-
age to make predictions, as many historians have claimed,
but that they did not possess the appropriate philosophical
attitude regarding the nature of the elements.
These days any high school student is likely to open a
textbook and find STM images of atoms and molecules. He
or she might then be perplexed by hearing the debate sur-
rounding the reality or otherwise of atoms that has taken place
over the years. However, as chemistry becomes increasingly
sophisticated the need for philosophical analysis of all aspects
of the subject, including perhaps the true significance of STM
images, will continue to increase. As someone once said,
philosophy begins with the recognition of the difference
between appearance and reality.4

4 Journal of Chemical Education • Vol. 77 No. XX Month 2000 • JChemEd.chem.wisc.edu
Black Boxes in Chemical Education
Finally, the direction taken by contemporary research in
chemical education has been cause for concern for some of
us. Instead of focusing on the content of chemistry courses,
the new tendency is to reach for cognitive psychology and
allied fields and to concentrate almost exclusively on the
“learning process” (15). The message appears to be that we
mistakenly regard the mind of the student as a form of black
box into which new information can be transferred intact by
the teacher. The new approach is to explore this cognitive
black box and to regard it as playing the central role in chemi-
cal education. But the enthusiasm with which this program
has been embraced has meant that we are rapidly approaching
the point where the content of chemistry courses is regarded
as a new black box, which is therefore ignored while all
attention is focused on how students learn.
I hope that a new interest in philosophy of chemistry
may provide the impetus required in chemical education to
redress the balance of focus back toward chemistry itself.
Notes
1. These journals are called Foundations of Chemistry and Hyle.
The author is editor-in-chief of Foundations of Chemistry, which invites
submissions on philosophical, historical, educational, cultural, and
conceptual aspects of chemistry. See Web page at http://www.wkap.nl/
journals/foch.
2. To subscribe to philchem send email message to listserv@
vm.sc.edu and write just the following in your subscription message:
subscribe philchem your name.
3. There is no implied connection with reduction in the sense
of the opposite of oxidation!
4. This issue has recently surfaced again, with the claim that
atomic orbitals have been directly observed.
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University Press: New Brunswick, NJ, 1992. Nye, M. J. From
Chemical Philosophy to Theoretical Chemistry; University of Cali-
fornia Press: Berkeley, 1993. Laszlo, P. La Parole des Choses; Col-
lection Savoir-Science: Paris, 1993. Hoffmann, R.; Laszlo, P.
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