Holly Vande Wall
Natural Kinds of Chemistry
A Critique of Brian Ellis’ New Essentialist Ontology
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Natural Kinds of Chemistry
A Critique of Brian Ellis’ New Essentialist Ontology
I. Introduction
Brian Ellis, in his recent book Scientific Essentialism, and its companion The Philosophy of
Nature: A Guide to the New Essentialism argues that certain essential properties of objects in the
world not only determine the nature of these objects but also how they will behave in any
situation. This project is an ambitious entry into the metaphysical discussions of natural kinds
and the laws of nature. Ellis believes that our current scientific understandings of physics and
chemistry suggest that our ontology should be built of irreducible properties, necessary
connections and a modernized version of Aristotelian essences. He argues that scientific realists
have every reason to regard inanimate objects in the physical world not as passive receptors of
“the forces of nature” but instead as active agents. “Things behave as they do,” Ellis suggests,
“not because of any external constraints that force them to, but because this is how they are
intrinsically disposed to behave in the circumstances.” (Ellis, p. 3) His main target is the
‘Humean’ ontology that has dominated 20th century analytical philosophy. “For a Humean, there
are two serious problems about the laws of nature: the necessity problem (to explain the nature,
or the illusion, of physical necessity) and the idealization problem (To explain the existence of
laws that appear to range over idealized systems, e.g. closed and isolated ones.)” (Ellis 2002,
123) In explaining how scientific essentialism eliminates these problems, Ellis engages a
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number of active philosophical debates, including issues of modality, induction, and realism, and
responses to his work from all of these areas are available in recent literature1
In this paper I will critique Ellis’ essentialism from a new perspective, that of the philosophy of
chemistry. I believe such a project is warranted because one of the characteristic parts of this
metaphysical project is its reliance on examples from chemistry. Historically, philosophers of
science tend to focus on physics as the paradigmatic science, and more recently philosophy of
biology has come into vogue. But Ellis not only uses chemical elements and molecules as
paradigmatic examples but expresses his hope that one of the main philosophical implications of
his project would be “the development of a realistic logic of chemistry…that did justice to the
complexities of the subject matter, and which chemists themselves would find useful.” (Ellis
2002, p. 139) I will argue that our current knowledge of chemistry in fact does not lend itself to
essentialist interpretations and that this seriously undercuts Ellis’ project.
II. Background to the “New Essentialism”
The modern debate over essentialism has its roots in John Locke’s rejection of the Aristotelian
explanation of natural kinds. Aristotle believed that the definitions by which natural objects
could be properly categorized were “ontologically correlated” to what was essential to each
object. The proper definition was that which captured the essential reality of that natural kind.
Locke could not be certain whether or not individuals possessed real essences, in the sense of
some microscopic and thus unobservable feature by which they could be grouped into natural
On modality Ellis obviously engages with the work of Saul Kripke and Hilary Putnam; for a critique of Ellis’ work
on the subject of modality see Lange. A ‘Humean’ response to this project is found in Helen Beebee’s brief but
excellent review of Ellis’ recent books. For the issue of realism regarding the laws of nature, Armstrong and
Mumford provide the most direct responses to Ellis’ work. Ellis actually includes and responds to Armstrong’s
objections in the appendix to The Philosophy of Nature.
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kinds. But he was certain that humans are only capable of categorization on the basis of
observable, macroscopic qualities. Thus Locke proposed that only nominal essences are
accessible to the human mind. Hume took Locke a step further, denying even the existence of a
real essence, and the analytic tradition followed in their footsteps.
In the 1950s students of modal logic, including Saul Kripke and Hilary Putnam, revived the
debate over essentialism. Scientific investigation had come a long way since Locke’s day –
scientists in the 20th century had access to microscopic features the 17th century couldn’t have
dreamed of – and those studying subatomic structure certainly seemed to be probing the “real
essence” of nature. Moreover, “Locke’s objection to essentialism had been that we cannot use
unknown criteria to organize things into kinds. Putnam suggested that perhaps we could do
precisely this, by appealing to ‘same kind’ relations.” (Reid, p. 70) People with little knowledge
of science could wrongly assign natural objects, thus separating the real from the nominal
essence, but such errors did not preclude the possibility of linking nominal and real essences
through natural kind terms.
Among the most important new defenders of the existence of essential properties and the reality
of natural laws is David Armstrong. He argues that “behind the set of ‘surface’ properties there
is a set of more deeply hidden properties: the real essence of the natural kind. The laws which a
thing obeys are in a sense part of its essence.” (Armstrong 1978, p. 66) Note that these essential
properties are not individuating properties – objects that have the same essential properties are
members of the same natural kind. This idea is the basis of Armstrong’s defense of universals.
“I would see the claim that there are universals as no more than the claim that different things,
different natural things, can have the very same properties and relations, with ‘same’ used in a
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perfectly strict sense. A special realm of universals is not required.” (Armstrong 1999, p. 84) For
example, if all members of the natural kind known as “a helium atom” can be defined as those
things which share the same essential property of having two protons in its nucleus then we
needn’t postulate a Platonic realm of forms to believe that there is a universal definition of what
a helium atom must be.
Armstrong and Ellis can both be described as essentialists about natural kinds. Where Ellis
diverges from Armstrong is in his description of natural laws. Armstrong argues that the laws of
nature are best understood as interactions between universals. Such interactions are contingent
upon circumstances but necessary if the right circumstances are in place.2 Armstrong is an
essentialist about natural kinds but a non-essentialist about the laws of nature. For Ellis,
however, the laws of nature are more than epistemological conveniences – they are ontologically
necessary, for reasons that will become clear in the next section. It is at this point that most
metaphysical critiques of his work are applied.3 I believe, however, that it is at the level of the
essential properties, not the natural laws that they purportedly cause, that a critique of Ellis’ work
should begin.
III. A Brief Introduction to Ellis’ Essentialism
As noted above, this philosophy of nature is rooted in a belief that there are internal properties of
natural objects by which they can be divided into categories and from which all their properties
and interactions can be explained. Ellis wants to jettison what he calls a passivist view of nature
in which “inanimate things are capable of acting only as directed – depending, for example on
how they are pushed or pulled around by God, or by the forces of nature.” (Ellis 2002, p. 1) But
2
See Mumford 2004 and Carroll for critiques of this view.
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this is a new essentialism, and Ellis must strike a balance between distancing himself from both
the Newtonian mechanist and the Aristotelian. One of the things which distinguishes new
essentialism from Aristotle’s, Ellis suggests, is “a much stricter conception of natural kinds…any
two members of the same natural kind must be identical in all essential respects…essentiality
implies necessity in the strict sense of metaphysical necessity.” (Ellis 2002, p. 14) For Ellis
there are no biological essences, individual essences or final causes. Aristotle might argue that
rationality is an essential property of humans and yet suggest that an individual human was
irrational. This is not permissible in the new essentialism, which is rarely concerned with
individuals in the ‘member of a kind with its own distinctive qualities’ sense. There is no room
in the new essentialism for a description of Brian Ellis as an individual with a mind/soul essence
and a human with a rational essence. On Ellis’ scheme, biological species are not natural kinds
because the differences between the members of species groups are too complex; they possess
too many individual differences for any essential qualities to be definable.4 He believes that this
kind of problem doesn’t arise at the chemical and physical level because scientific observation
suggests that objects at the atomic level have only one truly individuating property – position –
which is not an essential property. “The world evidently consists of vast numbers of things
belonging to intrinsically exact similarity classes, the members of which have exactly similar
intrinsic properties, and participate in exactly similar events and processes. I took the view that
these similarity classes must reflect a natural kinds structure of the world.” (Ellis 2005, p. 77)
There are, of course, other ways that any two atoms, or molecules, or whatever can be
distinguished. Two molecules might be isomers, or two atoms possess different numbers of
3
See especially Mumford and Armstrong
He does believe that cells and cellular processes have essential properties and that “there is no reason to think of
microbiological or biochemical processes as being fundamentally different from the processes occurring in non4
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neutrons. But these options are quite limited in relation to the huge numbers of atoms of that
type in existence. Therefore, at any given time there should be a very large number of objects
with all the same physical properties which otherwise differ only in position. Thus there is no
essential individuation at the atomic level. Ellis, like Armstrong, believes in universals.5
But as noted above, Ellis disagrees with Armstrong about the metaphysical necessity of the laws
of nature. For Ellis, the concept of metaphysical necessity has two key properties.
First, a metaphysical necessity is a genuine necessity. If something is metaphysically necessary,
then it must be the case, and there is no possible state of affairs in which it would not be. Not
even God, if there were such a being, could create a world in which anything that is
metaphysically necessary is false…Secondly, metaphysical necessities have to be discovered by
scientific investigation. They are not discoverable, as other necessities are, just by considering
meanings. (Ellis, p. 17)
The second half of this description is particularly important. Essential properties on this
definition are those properties that are discovered by scientific investigation to be the necessary
and sufficient conditions by which objects are grouped into natural kinds. Science, not logic or
linguistic analysis, is the final arbiter on this issue. To distinguish metaphysical necessity from
logical and linguistic necessity Ellis gives the common example of water and H2O. He argues
that the equivalence of these two is not a matter of human category choice. It is possible that
humans could have failed to discover that H2O and water are the same thing, but that it is
nonetheless metaphysically necessary that this is the case. Ellis quotes Hilary Putnam’s example
of a twin earth on which there exists a substance with all of H2O’s properties, which is also
called water, but is actually structure XYZ. He then suggests that because for essentialists “the
laws of nature are not contingent, but metaphysically necessary…their necessity derives from the
essential natures of the things on which they are said to operate.” (Ellis, p. 37) Thus the
living matter.” (Ellis, 154) As I don’t believe that chemistry can be meaningfully reduced to physics I am also
doubtful that biological processes can be understood as reducing to chemical ones, but I will not address that here.
5
Note the similarity of this description to Armstrong’s definition of a universal.
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macroscopic properties of water must be dictated by the metaphysically necessary properties of
the H2O molecule. There are serious difficulties with the proposition that it is metaphysically
necessary to connect H2O and water, which I will address in the next section.
Before I turn to critique, however, a few more definitions from Ellis’ project need to be made
clear. Ellis defines three types of natural kinds, the first quite a traditional definition and the
second two distinctive to his project:



Substantive – divisions of physical things, some more fundamental than others, that can
be subsumed into broader categories arranged in a hierarchical structure. E.g. an electron
is a natural kind, but so is an atomic particle and the latter includes the former as one of
its types.
Dynamic – processes such as diffraction, ionization, and vaporization are natural kinds
within this ontology
Property – natural property kinds include both particular (spherical, mass m, charge q)
and the broader kinds(shape, mass, charge)6
One final distinction is key to both Ellis’ project and my critique of it. In his third chapter, Ellis
devotes considerable space to explicating a compound distinction: between intrinsic and extrinsic
properties and essential and accidental properties.


The intrinsic/extrinsic property distinction - Intrinsic properties are roughly the same as
Locke’s primary qualities, “the properties of the objects themselves” as Ellis puts it.
Extrinsic properties may interfere with our perception of the object but they should not
figure into any definition of natural kinds. Consider the distinction already discussed in
regards to individuation. Position is considered an extrinsic property while atomic
number and molecular weight are intrinsic.
The essential/accidental distinction is required to describe an important difference
between the fundamental particles of physics and atoms and molecules at the chemical
level. “For the simplest natural kinds, the intrinsic properties are all essential properties.
For natural kinds at the atomic, molecular and higher levels, the intrinsic properties and
structures always include some states that are accidental.” (Ellis, p. 55) For example, a
chlorine atom whose electrons are in an excited state is as much chlorine as one whose
electrons are in their ‘normal’ orbitals. This is an intrinsic accidental property.
Beebee is particularly helpful in clarifying Ellis’ description of the types of natural kinds and this is largely drawn
from her review.
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Ellis uses this distinction to make it clear that while intrinsic properties can be both essential and
accidental properties, no extrinsic properties are essential.
In summary, this new essentialism is an ontology in which the laws of nature and the properties
that define natural kinds are necessarily connected. There is a hierarchy of natural kinds of
objects whose members must share certain essential properties and may or may not share certain
accidental properties. It is the job of science to observe the behavior of objects and thus discover
what the properties of objects are and to distinguish the accidental from the essential. Within
this ontology, natural laws are descriptions of how objects are intrinsically disposed to behave.
IV. Critique of Scientific Essentialism from Chemistry
In this paper I suggest that Ellis’ examples of essential natures from chemistry do not conform to
our best scientific understandings of the chemical world. Ellis thinks his new essentialism ought
to appeal to chemists, and on a surface level it does. Chemists have what Ellis calls the most
obvious examples of natural kinds – the elements – and of the utility and predictive power of
categorization into natural kinds – the periodic table. The laws of periodic regularity provide a
clear example of how intrinsic properties of elements affect their outward behavior. Probing
deeper, however, it becomes clear that chemical elements and molecules are not good candidates
for natural kinds in Ellis’ sense. His paradigm examples of metaphysical necessity break down
under scrutiny. Many of the properties Ellis suggests are essential - shape, atomic weight,
atomic number, electron affinity – have serious problems. Whether any given property should be
considered accidental or essential, and whether only internal properties are essential seems to
vary depending upon which natural kind is under discussion.
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First, consider the concept of metaphysical necessity. Ellis defines a metaphysical necessity as a
state of affairs that could not be otherwise than it is and must be discovered by scientific
investigation. As an illustration of metaphysical necessity, Ellis quotes Hilary Putnam’s example
of a twin earth on which there exists a substance with all of H2O’s properties, which is also
called water, but is actually structure XYZ. Putnam uses this example to make a point about
modal relationships in possible worlds, and it is not my purpose here to critique his theory of
modality. More interesting for my purposes is the widespread use of Putnam’s example, which
makes it clear that most philosophers are under the impression that water is nothing more than a
bunch of H2O molecules. This is simply not the case. An individual molecule of H2O doesn’t
have any of the observable properties we associate with water. A glass of water, pure as water
can be, is better understood as containing H2O, OH-, H3O+ and other related but less common
ions, and even this is a vast oversimplification.7 Our current best understanding of the electron
transfers that give water the properties we observe is a statistical average of ever changing
interactions so complex as to be quite literally unthinkable. Indeed, the problem is “not that we
are unsure which (distribution of types of) microstructure is the correct one. The point is that
there is no one correct microstructure, because the microstructure depends as much on the
context and functions just as another nominal essence would.” (van Brakel, 2000b, p. 80-81)
This is why chemists use the ‘mixture of ions’ model to describe water’s macroscopic behavior.
The only thing we can say about a glass of water that is not, strictly speaking, an error is that the
average ratio of atoms in the glass is 2 H: 1 O and that it has the macroscopic properties of water.
If there are other kinds of atoms in the glass, or if the ratio is other than 2:1, then we do not have
pure water. If the ratio is 2:1 but it does not have the macroscopic properties (pH, boiling point,
7
If we could get truly pure water, which we can’t.
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etc.) of water then we have not water but a mixture of hydrogen and oxygen molecules in their
elemental form. Chemical analysis and experiment deals with water not as an individuated
molecule but in macroscopic quantities. All the typical observable properties of water – its pH,
its density, its boiling and freezing points, its utility as a solvent, are dependent not upon its
atomic ratio but the interactions between the dissociated ions.8 Philosophers of chemistry have
been arguing this point for at least 25 years; the fact that Ellis can blithely use this example in his
earliest chapters suggests that they either aren’t being read or haven’t been understood.9
Moreover, it is important to note that water is by no means unique in this regard. If a molecule is
defined as “the simplest structural unit that has the characteristic physical and chemical
properties of a compound” as it is by Webster’s dictionary, then no dissociating liquid, metal,
salt, or electrolyte fits the definition. Jaap van Brakel argues that “the concept of molecular
structure seems to derive its meaning more from the way molecules are represented in models
than from anything else. [The idea of a] Molecule is an indispensable, but thoroughly theoretical
concept; it is part of theories that are impressively empirically adequate, but without giving a
clear idea of what entities are thought to exist.” (van Brakel 2000, p. 174)
All this is meant to establish that if only science can establish what the metaphysically necessary
properties of water are, we know that “being H2O” is not one of these properties. We must ask
again, what the real essence of water is. Just what properties and structures are metaphysically
necessary? In seeking possibilities we must consider Ellis’ distinction between intrinsic and
extrinsic properties and essential and accidental properties. A metaphysically necessary property
8
9
Plus, at high temperatures water dissociates into two separable constituents. See van Brakel 2000a, p. 173.
See especially Primas, 1983
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must be neither extrinsic nor accidental. “Essential properties distinctive of a natural kind,” Ellis
reminds us “must be independent of the histories, locations and surroundings of its members.”
(Ellis 2002, p. 51) To clarify this point, Ellis notes that where uranium was mined has nothing to
do with the essential properties that make it uranium. But the case of water makes this
distinction less obvious. In order to behave like water, the H2O molecules and ions must be in
contact. Compare this to the uranium mining example – here it seems that where the molecule is
found (in contact with other molecules and ions of oxygen and hydrogen and not, for example,
isolated in the interstices of a crystal structure) is an extrinsic quality. But it is nonetheless
essential to for it to have the properties of water.
To solve this difficulty, we might consider that substances are not the only natural kinds
available in Ellis’ ontology. The metaphysical necessity might apply not to the substance of the
individual H2O molecule but to the dynamic natural kind of dissociation. It is a property of
members of the dissociating natural kind of molecules of which H2O is a member to form a mass
of ever changing ions if such molecules come into contact with each other. What is not clear,
however, is how the intrinsic/extrinsic distinction functions in the case of dynamic natural kinds.
It is difficult to conceptualize what intrinsic means when applied not to an atom or a molecule
but to the dynamic processes of ionization or vaporization. This is a serious issue for chemistry,
since “the study of the dynamics of molecular interaction” serves as a definition of fully half the
field. I will have more to say about the problems of dynamic systems for scientific essentialism
in the fifth section, on the problems of reducing chemistry to physics.
For the moment, I will remain within the domain of the substance natural kinds. Recall Ellis’
example of recently mined uranium. If the electrons of the uranium were in an ‘accidentally’
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excited state, this would clearly not impinge upon its essential status as an element. Where the
essential/accidental distinction becomes less easy to draw, however, is with intrinsic qualities
like the number of neutrons in a nucleus. Uranium 235 and Uranium 238 are, in one sense, both
‘essentially’ uranium, as they share the same atomic number. But the extra neutrons give the
atoms quite different properties – one is intensely radioactive and the other is stable. It is unclear
in this case whether having a particular number of neutrons constitutes an accidental or an
essential intrinsic property. This is a serious problem, not just for radioactive elements. Ellis
argues that “the chemical elements and compounds constitute the most readily accessible system
of natural kinds of substances, their properties are mostly their essential properties, and the
processes they undergo in chemical interactions are all natural kinds of processes that display the
essential properties of the substances involved.” (Ellis 2002, p. 139) Atomic and molecular
weights would seem to be among the most obvious candidates for essential properties. They
affect the properties of the macroscopic substance and were the original means by which the
elements in the periodic table were ordered.10
So what are the essential properties of the natural kind, H2O? Molecular weight is a problematic
choice, because there are multiple weights for this compound, due to the existence of isotopes of
both hydrogen and oxygen. Molecular weights of anything from 18 to 24 are possible given
various combinations of the isotopes. It is possible that we could define each isotope of an
element as a separate natural kind, but chemists, have chosen not to do so.11 Oxygen atoms with
weights of 16, 17 and 18 are referred to as the same element. It could be argued that this is just a
linguistic convention favored by chemists; in which case isotopes do constitute distinct natural
10
But this is not how we order the elements today. Now it is done by atomic number. The weight of element 52,
Tellerium is greater than that of element 53, Iodine.
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kinds. On this picture the element oxygen is the natural kind of which the 18O isotope is a
subset. Molecular weight would thus be an essential property of the natural kind ‘isotope’, but
not of the natural kind ‘element’. In favor of this view we can consider the isotopes of hydrogen.
Because the addition of a neutron more than doubles the weight of the nucleus, this “heavy
hydrogen” has distinctly different properties than the more common hydrogen atom and even has
its own name, deuterium and its own chemical symbol, D. The boiling point of D2O differs from
H2O by four degrees Celsius.12 But D does not have its own place in the periodic table. Chemists
have very good reasons for the convention of referring to isotopes of an element as the same
natural kind – with the exception of hydrogen, isotopes of elements have identical chemical
properties. They form the same compounds, exhibit the same electrochemical properties, have
the same pH in solution and so on. It is notoriously difficult to isolate the isotopes of an element
using chemical methods.13 The regularity and predictive power of the periodic table depends not
on atomic weight but on atomic number, meaning the number of protons in the nucleus of an
atom. The essence of the definition of the element oxygen is a nucleus containing eight protons.
My point here is that if we are observing natural laws in order to understand what the essential
properties of objects are and thus sort objects into natural kinds then we encounter a serious
difficulty with the hierarchy of natural kinds. If the essential properties of an isotope are
different from the essential properties of an element then what are the essential properties of a
molecule? H2O and D2O are made of essentially the same elements but not of the same isotopes.
It would seem that which properties chemists will deem essential to explain an object’s behavior
11
Although not without debate; for a much more detailed an historical account of the difficulty of defining chemical
elements, especially as it relates to the problem of reducing chemistry to physics and chemical realism, see Scerri.
12
There also exists an isotope of hydrogen with two neutrons – tritium - which has yet more distinctive properties.
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depend upon what behavior they happen to be observing. If you are interested in solubility, H2O
and D2O are the same molecule. But if you are measuring boiling points it is clear that the
essential property is actually atomic weight and you are dealing with two quite different
substances.
Given the difficulty of identifying metaphysical necessity based on structural and dynamic
properties, we might focus on natural property kinds, such as shape, mass and charge. After all,
this discussion of the nucleus of the atom is somewhat misleading. Chemical reactions are, by
definition, reactions between the electrons of elements and compounds. Interactions at the
nuclear level are formally the domain of physics. The chemical properties of an atom are the
result of the nucleus’ relative affinity for its electrons and their placement in the purely
theoretical spaces called orbitals. If we return, once again, the H2O molecule, we find that many
of the macroscopic properties of water are at least partially attributed to the “bent” shape of H2O,
which helps create the phenomenon of hydrogen bonding. Ellis believes that substances have an
essential shape to which they always want to return, although circumstances may distort the
shape we observe. He provides the example of a rubber band. A stretched rubber band has a
shape but this is not the “intrinsic shape” of the band; “its actual shape is best considered as
being made up of two distinct parts: an underlying intrinsic shape, and a superimposed
distortion.” (Ellis 2002, p. 52) However, in his article on the distinction between primary and
secondary qualities in chemistry, Jeffry Ramsey argues that while the shape of molecules is
explanatory of physical properties, it is not an essential property. He notes that while shape is
generally referred to as an essential quality by philosophers of science, chemists are quite clear
that molecular shape is not an essential property. The best explanation we have for chemical
13
The most famous example being the enormous technical achievement (not to mention expense) required to
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properties is that molecules are in constant, internal motion – the bonds twist, lengthen, contract
and bend – and the degree to which they do this depends upon the context they are in. Ramsey
concludes that “philosopher and scientist cannot and should not rely uncritically on the claim that
microstructure is an essential property of substances…a particular shape does not ‘belong’
essentially to matter as a ‘basic’ physical feature. Whether a particular ‘structure’ can be
imputed to a sample is a contingent matter.” (Ramsey, p. 122)
In light of Ramsey’s work the question for Ellis is: what basis do we have for designating one
particular shape as “intrinsic” while all others are merely “superimposed distortions?” It is
possible to isolate an H2O molecule within a crystal lattice structure and supercool it, with the
result that its bonds twist, contract and bend very little. If this is the intrinsic shape of the H2O
molecule then it is hard to understand how the laws of nature which build up the macroscopic
properties of water from the intrinsic essential properties can arise from it – because this is not
the shape that the H2O molecule has in contexts where it is neither isolated nor supercooled.
This is a distinctly unnatural state for an H2O molecule on earth. Suppose we choose instead to
analyze the shape of any single H2O molecule at 20C, just one molecule, at one instant of time,
in a bowl full of liquid water.14 But then we have the problem that most of the molecules in the
bowl are actually ions and all of them have bonds in motion. Either selection seems entirely
arbitrary. One option is to say that the object has “a shape” but not “a particular shape” but this
cannot be what Ellis means, because that would make shape explanatorily uninteresting15, and
Ellis expressly selects shape as his paradigm example of a “causally intrinsic” property. A
possible response is that for us to choose an intrinsic shape is arbitrary but this does not mean
separate Uranium 235 from Uranium 238 for the production of atomic weapons.
14
And keep in mind that the act of examining the molecule’s shape, through whatever form of spectrographic
analysis, will immediately distort it.
16
such a shape doesn’t exist, only that science cannot determine precisely what it is. The problem
with this suggestion is that Ellis defines metaphysically necessary properties as those that science
can discover. If science in principle cannot know what the intrinsic shape of water is, then shape
cannot function as a metaphysically necessary property within scientific essentialism.
When we examine Ellis’ defense of intrinsic qualities it seems that Ellis is embracing an even
more controversial stance; he moves from a scientific model to a metaphysical necessity. In
defending the concept of causal intrinsicality, Ellis argues that it “is not only required for an
understanding of essentialism, but it is also one that has a fundamental role in scientific theory.”
He gives two examples of this “model-theoretic” type of scientific explanation.
“To explain the shape of the earth, for example, we must contrast its actual shape with the shape it
would have theoretically if it were not distorted by tidal forces, not rotating on its axis, not
vibrating and so on…To explain the motion of projectile…we must analyze the motion to be
explained as one that has both a natural part and a forced part. The natural part, which requires no
force to sustain it, is assumed (or has been since Newton) to be a uniform straight-line motion.
The forced part is then calculated as the difference between the actual motion and the supposed
natural motion.” (Ellis, p. 53)
It is somewhat odd that Ellis uses an example from the Newtonian mechanist world view, which
is the very position he’s trying to oppose, as an example of the kind of scientific theory best
preserved by essentialism. Be that as it may, it is far from clear that Newtonian motion is in any
way “natural” since it leaves out all of the natural variables involved in actual motion. When our
experience contradicts mathematical idealizations like Newton’s ideal of projectile motion, one
option is to attribute the ‘error’ to moderating effects - friction, the medium, etc. The other, just
as viable possibility, is that nature is simply not in conformity with ideal constructs. On this
view, the picture of a perfectly spherically shaped earth, like a picture of a bent H2O molecule, is
an inaccurate statement we knowingly make in order to simplify our calculations. Statistically,
15
This suggestion is also part of Jeffry Ramsey’s article.
17
the average of a vast number of H2O molecules in various shapes seem to behave as if they were
bent to a certain angle with bonds of a certain length, twist, etc. But it is far from obvious that a
mathematical idealization of a far more complex actual behavior should qualify as our basis for
belief in a metaphysically necessary property. I would suggest that the “bent molecule” picture,
like the round earth and Newtonian projectile motion, is better viewed as a scientific model – a
convenient fiction of scientific theory – than an essential property. To argue that the theoretical
construct of a perfectly uniform body moving in perfectly frictionless space has more
metaphysical reality than any actual body moving in actual space will not be the most common
intuition among the scientists who utilize “model-theoretic patterns of explanation.”
V. Why Physics Alone isn’t the Answer – Problems of Reductionist Essentialism
I have argued above that while chemical elements and molecules and processes provide Ellis’
primary examples for natural kinds, most of the obvious candidates for essential properties of
these natural kinds have significant problems. One answer to my critique might be to deny the
chemical properties are essential, and constrain essentialism to sub-atomic physical properties.
After all, Ellis argues that “for the simplest natural kinds, the intrinsic properties are all essential
properties. For natural kinds at the atomic, molecular and higher levels, the intrinsic properties
and structures always include some states that are accidental.” (Ellis 2002, p. 55) Several
difficulties I identified in chemistry center around the problems of distinguishing accidental from
essential properties. At the sub-atomic level, however, Ellis believes this problem is eliminated.
Perhaps all that is needed to ground an essentialist ontology is to reduce chemistry to physics.
18
The problem with this argument is that it is not as easy to reduce chemical properties to quantum
or statistical mechanics as physicists have tended to suggest.16 Certainly such reduction is
currently impossible given the sheer size of the calculations involved.17 But there remains
considerable debate as to whether chemistry is even in principle reducible to physics. Primas
argues that if by reduction we mean “higher-level theory together with its interpretation can be
deduced from the basic theory” then chemical kinetics cannot be derived from quantum
mechanics, the theory of heat has not been reduced to statistical mechanics, temperature is not a
molecular concept, the model of hybridization of orbitals is not predicted by quantum
theory…and so on. (Primas 1991, p. 163) The literature on the reduction of chemistry to physics
is vast and inconclusive.18 For my purposes, however, the most important point is the problems
this raises for essentialism. Ellis admits that the two key problems for essentialism are:
how are natural kinds to be identified, and how are their essential natures to be discovered?
…There can be legitimate doubts about whether two things are members of the same natural kind,
or whether the properties or structure that have been postulated as essential really are
essential…But these are the kinds of doubts and concerns that working scientists are accustomed
to, and know how to handle. They are not irresolvable skeptical doubts like those generated by
Humeanism. (Ellis 2002, p. 135-136)
Among the more famous statements of this kind comes from Paul Dirac: “the underlying laws necessary for the
mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the
difficulty is only that exact applications of these laws lead to equations which are too complicated to be soluble.”
Other proponents of chemistry’s reduction to physics include Oppenheim, Heisenberg, and Reichenbach. (See von
Brakel 2000a)
17
To grasp the level of complexity involved, consider that “for a molecular structure calculation, all that is really
required is to determine the arrangement of electrons that corresponds to the particular fixed geometric arrangement
of nuclei that represents the stable geometry of the molecule.” Note, however, that we haven’t managed to do this
for any molecule much larger than CH4. “To perform a similar calculation for a dynamic problem, electron
arrangements would need to be calculated for a representative set of all the of the different possible geometric
arrangements of nuclei that are likely to be encountered in the course of reaction.” For a reaction of just three atoms
this is already 1000 times more difficult to calculate that the structure of any one atom. For a four atom reaction the
calculation goes up by a factor of four, for five atoms the by a factor of four again, so that a reaction between five
atoms (which is a very small reaction indeed) is one billion times more complicated than the structural determination
of a single atom. Many chemists believe that if we actually did the quantum calculations for dynamic systems they
would have predictive power. But we have no empirical data for this, only an argument from analogy. (Christie, p.
46)
18
For a review of the debate, see van Brakel 2000b, especially chapter five.
16
19
I have two objections to this: first, most “working scientists” are much less interested in the
essential natures of things than whether or not their models of how nature behaves yield the
results they desire. The identification of natural kinds is much more likely to be a philosophical
project than a scientific one. Second, and more importantly, these “kinds of doubts and
concerns” sometimes do seem to be irresolvable, for reasons that are less metaphysical than
epistemological. Whether we define H2O and D2O as members of the same natural kind depends
upon what properties of these molecules we wish to investigate. Organic chemists, examining
the interactions between the billions of molecules in an aqueous solution, will define the essential
properties of water as a natural kind quite differently from a physical chemist studying individual
H2O molecules in crystalline structures, and both will differ from a physicist studying which
types of quarks make up the proton. Because different fields focus on different properties of the
natural world, debate over whether isotopes or elements are the more fundamental natural kind
began in the early years of the 20th century and has not been decided today. The most
fundamental ordering of chemistry - the periodic table - is not derivable from quantum theory.
(See Scerri) Mendeleev’s periodic law is a marvelous tool for ordering the chemical elements
based on atomic number, but it does not accurately predict atomic weights. Neither can it
suggest which is the more essential property. Again, it depends upon what we want to know.
Ellis has turned the two most difficult questions in the formation of his ontology over to science
but it is not clear that scientists particularly interested in the question and, where they are
interested, that they are capable of providing the answers.
20
VI. Conclusion
There are elements of this new essentialism that make it a very tempting theory for philosophers
of chemistry. Ellis takes scientific research as the font information about the structure of the
natural world. He offers a simple account of natural laws from within causal realist metaphysics.
Chemists are comfortable with the essentialist idea that the internal properties of a molecule are
necessarily connected to the law-like behavior it exhibits. But the key word here is “law-like.”
The “laws of chemistry” are approximations based on models the chemists know to be
oversimplified.19 Scientific essentialism relies on scientists to identify natural kinds and the
essential properties of natural kinds, and a sophisticated examination of chemical properties
shows just how difficult this is to do. Macroscopic properties of substances are not easily
reduced to their microscopic properties. The metaphysically necessary properties of water
cannot be encompassed by an individual H2O molecule. Ellis believes that no extrinsic quality
has causal power, while I suggest that where an object is found seems to have a very significant
causal effect. Philosophers of chemistry are not disposed to accept molecular shape as an
essential property, while Ellis selects it as his paradigm example of a causal intrinsic property.
At least in the case of the water and H2O molecules, it seems that Ellis has wrongly assigned
which are accidental and which are essential properties, largely because he has equated the
macroscopic properties of one with the microscopic properties of the other. This ought to be a
very worrying mistake for someone attempting to guide us into the new essentialism.
19
For a detailed discussion of the odd status of laws in chemistry, see especially Christie and Scerri
21
If essential properties only exist at the level of sub-atomic physics, then Ellis’ hopes that
essentialism will provide a theoretical basis for a philosophy of chemistry are at best hopes for a
very distant future. The argument that chemical structure and dynamics can be explained at the
quantum level derived is purely from analogy to much simpler systems than those chemists
actually study. All of this suggests that we have very little scientific evidence that scientific
essentialism is a viable ontology. Many of these same critiques apply to Armstrong’s description
of universals, because he and Ellis share key assumptions about the existence of essential
properties.
I do not think, however, that these problems commit us to a full-fledged idealism about the
natural world or a deep Humean skepticism about induction. Instead, they suggest that science is
a better guide to epistemological than metaphysical characterization of the world. Philip Kitcher
has emphasized the ways in which our categorization of objects depends upon our particular field
of interest. This isn’t to say that the world is mentally constructed in the strong sense that
nothing exists without our perception of it. Instead, Kitcher suggests that
Insofar as we attempt to describe, to predict, to explain nature we are inevitably selective. Hence,
in providing a literal interpretation of ‘nature’s own language’ we are driven to the conclusion that
the languages we use are apt for the description of nature in the sense that they are good for
creatures like us to formulate the kinds of descriptions of the world that we care about. (Kitcher,
p. 46)
After more than one hundred years of debate, it remains unclear whether different isotopes of the
“same element” belong to the same natural kind. In practice, however, this ontological
difficulty has had little impact on scientific research. In some situations, such as in ordering the
periodic table, it is convenient to place the U235 and U238 together. In other contexts, such as
harnessing radioactivity, it is not. I suggest that in the end debating which properties of a
22
chemical element are essential seems a less valuable project for science than making note of
which properties are of interest in which context.
23
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