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available alleles almost always results in a viable reproductive individual.
This is what Eldredge, Gould, and Hull refer to as “homeostasis” in the
gene pool.
Underlying these stabilizing forces, however, is an even more funda-
mental force. New gene tokens are copied from old ones. A massive
replicating process is at work in the continuation of a species. The role
of the forces producing homeostasis is secondary, keeping the reproduc-
ing or copying relatively faithful over periods of time. The role of
homeostatic forces is to see that the kind does not do as Achilles™ horse
did and “run off in all directions,” but remains relatively stable in its
properties over time.
In sum, the members of biological taxa are like one another, not be-
cause they have inner or outer causes of the same ahistorical type, but
because they bear certain historical relations to one another. It is not just
that each exhibits the properties of the kind for the same ahistorical or
eternal reason. Rather, each exhibits the properties of the kind because
other members of the kind exhibit them. Inductions made from one
member of the kind to another are grounded because there is a
causal/historical link between the members of the kind that causes the
members to be like one another. Biological taxa are historical kinds.
I have mentioned that the ontological ground of induction for many
stuffs is ahistorical, for example, the ground of induction for the various
chemical elements and compounds is ahistorical. But there also are stuffs
whose ground of induction is historical, for example, peanut butter re-
tains its basic properties over encounters because it is what is made by
grinding up peanuts, which constitute a historical kind, and cowhide
does because it is the hide of the historical kind cow.
The two most obvious sorts of historical reasons why members of a
kind might be caused to be like one another are, first, that something
akin to reproduction or copying has been going on, all the various
members having been produced from one another or from the same
models and/or, second, that the various members have been produced
by, in, or in response to, the very same ongoing historical environment,
for example, in response to the presence of members of other ongoing
historical kinds. A third and ubiquitous causal factor often supporting
the first is that some “function” is served by members of the kind,
where “function” is understood roughly in the biological sense as an ef-
fect raising the probability that its cause will be reproduced, that it will

be “selected for reproduction.” It is typical for these various reasons to
be combined. For example, many artifact kinds combine these features.
Thus Frank Keil remarks,

Chairs have a number of properties, features, and functions that are normally
used to identify them, and although there may not be internal causal homeo-
static mechanisms of chairs that lead them to have these properties, there may
well be external mechanisms having to do with the form and functions of the
human body and with typical social and cultural activities of humans. For ex-
ample, certain dimensions of chairs are determined by the normal length of hu-
man limbs and torsos. . . . (Keil 1989, pp. 46“7)

Chairs have been designed to fit the physical dimensions and practical
and aesthetic preferences of humans, who are much alike in relevant re-
spects for historical reasons. Moreover, the majority of chairs have not
been designed from scratch, but copied from previous chairs that have
satisfied these requirements.They thus form a rough historical kind ow-
ing to all three of the above reasons. Clearly there are reasons that go
well beyond (mysteriously agreed on) points of definition why one
knows roughly what to expect when someone offers to bring a chair.
Similarly, one knows what to expect when someone offers to lend a
Phillips screwdriver (designed to fit screws that were designed to fit
prior Phillips screwdrivers), or to take one to see a Romanesque church
“ or, of course, to replace your back doorknob.3
The members of some historical artifact kinds are similar in nearly
the same detail as members of animal species. In Millikan (1984), I
spelled out why the 1969 Plymouth Valiant 100 was a “secondary

. . . in 1969 every ™69 Valiant shared with every other each of the properties
described in the ™69 Valiant™s handbook and many other properties as well. And
there was a good though complicated explanation for the fact that they shared
these properties. They all originated with the selfsame plan “ not just with
identical plans but with the same plan token. They were made of the same ma-
terials gathered from the same places, and they were turned out by the same
machines and the same workers . . . or machines similar and workers similarly
trained [on purpose] . . . [Hence all the Valiants] had such and such strengths,
dispositions and weaknesses . . . placement of distributor . . . size of piston rings
. . . shape of door handles. . . . Valiants, like most other physical objects, are

3 The reference is to Fodor (1998).

things that tend to persist, maintaining the same properties over time in accor-
dance with natural conservation laws. . . . Also, there are roughly stable prevail-
ing economic and social conditions . . . in accordance with which working
parts of automobiles tend to be restored and replaced with similar parts. . . .
[The Valiant also] has an identity relative to certain kinds of conditional
properties. . . . For example, the fenders of the ™69 Valiant that has not been
garaged tend to rust out whereas the body stands up much better; the ball
joints are liable to need replacing after relatively few thousands of miles whereas
the engine . . . is not likely to burn oil until 100,000 miles. . . . (Millikan 1984,
pp. 279“80)

Historical kinds of a somewhat less concrete nature are, for example,
retail chains (McDonald™s, Wal-Mart) and buses on a certain bus line
(bus #13, the Elm Street bus).4 Many kinds of interest to social scien-
tists, such as ethnic, social, economic, and vocational groups, are histor-
ical kinds. For example, school teachers, doctors, and fathers form his-
torical kinds when these groups are studied as limited to particular
historical cultural contexts. Members of these groups are likely to act
similarly in certain ways and to have attitudes in common as a result of
similar training handed down from person to person (reproduction or
copying), as a result of custom (more copying), as a result either of nat-
ural human dispositions or social pressures to conform to role models
(copying again) and/or as a result of legal practices. More generally, they
are molded by what is relevantly numerically the same historical niche,
a certain homeostatic ongoing historical social context that bears upon
them in ways peculiar to their social status. Boyd (1991) claims that
members of some social groups may exhibit properties characteristic of
the group as a result of being classified into these groups rather than
conversely, but he argues that this does not compromise these social
kinds as possible scientific objects. Members may come to form a co-
hesive social kind “only because” other members of the society class
them together (stereotyping, prejudice, taboos), but the “because” here
is causal, not logical, resulting in certain derived uniformities among
members of the group. The kind that results is then real, not merely
nominal. If social groups were not real, there could be no gain in em-
pirical studies concerning them, for example, studies of the attitudes of
American doctors toward herbal medicines, and so forth. Doctors are an
actual-world group, not a set of possible properties in a set of possible
worlds. That is why their attitudes and practices can be studied empiri-

4 The latter example is Richard Grandy™s (from conversation).

cally. On the othe hand, insofar as social scientists sometimes generalize
across radically different cultures, not just, say, across Western cultures,
the common historical thread across social groups is mainly just human
psychology, the common psychological dispositions of the historical
species Homo sapiens.
Historical kinds do not have “essences” in the traditional sense. On
the other hand, a kind is real only if there is some univocal principle,
the very same principle throughout, that explains for each pair of mem-
bers why they are alike in a number of respects. That is, the principle
explains the likeness between members, not, in the first instance, the
properties themselves. (To explain why a photocopy is like the original
is not to explain why either has the properties it has. I can know why
the photocopy is like the original without knowing what specific prop-
erties either of them has.) Only in some cases does the best explanation
of this likeness concern likeness in inner constitution. In the case of his-
torical kinds, although a statistically significant likeness among inner
constitutions may result from the principles that group the members
into the kind (most of your and my genes are the same5), this probab-
listic result is not what defines the species™ unity. Most real kinds do not
have traditional essences, but to be real they must have ontological
grounds, and these could, I suppose, be called “essences” in an extended
sense. One or another kind of glue must hold them together, making it
be the case that properties exhibited by one member of the kind are al-
ways or often exhibited also by other members, so that induction is
supported. We could extend the term “essence” so that it applies to
whatever natural principle accounts for the instances of a kind being
alike. But it is probably safer to stay with the term “ontological ground
of induction” to avoid any possibility of misunderstanding.


Not only real kinds but all substances must be held together by some
kind of ground of induction.That is what makes them substances. A sub-
stance is something that one can learn things about from one encounter
that will apply on other occasions and where this possibility is not coin-
cidental but grounded.There is an explanation or cause of the samenesses.

5 About 90 percent are likely to be the same. It does not follow that there are many
(even any) genes common to everybody. (To conclude so would commit the fallacy of

Ghiselin and David Hull said that species are “individuals” because
they are held together not by a traditional essence but through histori-
cal causal connections. The other side of this coin is that individuals are
rather like species: Their ontological ground of induction is similar. If
Xavier is blue-eyed, tall, good at mathematics, and intolerant of gays to-
day, it is likely he will be so tomorrow and even next year. This is be-
cause he too is a “homeostatic system . . . amazingly well-buffered to
resist change and maintain stability in the face of disturbing influences,”
and because Xavier tomorrow will be a sort of copy of Xavier today.
Xavier today is much like Xavier yesterday because Xavier today di-
rectly resulted from Xavier yesterday, in accordance with certain kinds
of conservation laws, and certain patterns of homeostasis, and because of
replications of his somatic cells. Ghiselin and Hull say that species are
individuals; conversely, some philosophers have thought of Xavier as a
class consisting of Xavier timeslices, each of which causes the next. Ei-
ther way, there is a deep similarity between individuals and many his-
torical kinds.
Because of the rich ontological ground of induction on which bio-
logical species rest, one can run numerous inductions over the members
of any species, learning about most members from observing one or a
few. The elementary student learns about sulphur from experiments
with one sample. Similarly, she learns about frogkind by dissecting one
frog, and about the human™s susceptibility to operant conditioning by
conditioning one friend to blink for smiles. One can learn from sample
members of a species about the whole species for much the same rea-
son one can learn about one temporal stage of a person from other
temporal stages of the same person, and vice versa.


Unlike eternal kinds, historical kinds are not likely to ground many, if
any, exceptionless generalizations. The copying processes that generate
them are not perfect, nor are the historical environments that sustain
them steady in all relevant respects. This is true of individuals as well.
Depending on the category of individual and what it is made of, some
properties will be less likely to change than others, but usually there are
very few that could not change under any conditions. The idea that ei-
ther a historical individual or a historical kind is somehow defined for
all possible worlds, not just this one, such that there are definite proper-

ties that must endure for the individual to remain in existence, or that
must be present for the kind member really to exemplify the kind, is
mistaken. Who is really and truly a member of the working class? Here
the principle or principles that cause or tend to hold the kind together
catch up some members more squarely than others. Was Theseus™s ship
still the same ship after its last plank was replaced? There is nothing in
nature to draw such distinctions. Historical kinds typically have natu-
rally and irreducibly vague boundaries. So do historical individuals. If
their boundaries happen to be sharp, as they sometimes are in practice,
this is a matter of historical fact, not some deeper necessity.
Real kinds are domains over which predicates are nonaccidentally
projectable.There are good reasons in nature why one member of a real
kind is like another. So, although real kinds can have vague boundaries,
still, the question whether an item belongs to a certain real kind or not,
or whether it is on its border, is written in nature, not just in English or
!Kung. Whether a seemingly marginal item is or is not a member of a
certain real kind often is a straightforward substantive question about
how the world is, not a question of how we humans or we English
speakers like to classify. If it is not like other members of the kind for the
very same reason they are like one another, then no matter how many
properties it has in common with them, it is not a member of the same
real kind. Similarly, we take it quite rightly that whether a correct iden-
tification of an individual has been made is a matter of how the world
is, not of how we humans or we English speakers like to identify. This
has not, of course, stopped philosophers interested in such questions
from thinking up numerous bizarre possible-world examples where it
would not be clear whether this individual thing would be numerically
the same as that one. Similarly, they might raise the question whether a
dog with, say, ¼ or ¹„‚… or ¹„‚‚ coyote genes spliced in would be a dog. But
the home of historical substances is in this world. Questions concerning
their identities in other worlds are, in fact, subtly incoherent.
Historical substances are not likely to ground exceptionless general-
izations. But many substances interest us not because they afford such
reliable inductions, but because they afford so many inductions. They
bring a great wealth of probable knowledge with them.6 This gives rise,

6 Andrew Milne suggests that historical kinds may be likely to have more projectible prop-
erties than ahistorical ones because “with historical kinds, often things that are nomically
quite separate are still projectible. . . . Properties that are only contingently correlated, in
the sense that it is perfectly lawful for one to occur without the other, may nonetheless be

presumably, to the typicality effects explored by contemporary psychol-
ogists studying categories. It seems natural that people should work
with a stereotype taken from knowledge of the most stable properties of
substances when asked to describe the substance, in making guesses
about category membership, when asked to make inferences about un-
observed members, and so forth.
Because the occurrence of causative factors accounting for similari-
ties can be more or less regular or irregular, and because the number of
grounded similarities characterizing a substance can be larger or smaller,
there are two different continua from richer to poorer along which his-
torical substances can range.These reflect (1) the reliability of the infer-
ences supported, and (2) their multiplicity. Substances vary widely in
both of these dimensions. If the substance is sufficiently impoverished in
both of these dimensions, whether there exists a real kind at all can be
a vague matter.There is no sharp line between what is and is not a sub-
stance. Rather, some things are, as it were, better substances than others,
some are worth understanding as substances, others are too marginal or
uninteresting. One might argue that even Californians form a very
rough or vague historical kind.They are of the same species, many have
copied behavioral patterns from one another, they have been subject to
certain social and physical environmental influences from the same
sources; hence, certain very rough and uncertain generalizations can be
made over them for good reason. There is a long, graded continuum
between historical kinds suitable, say, to project sciences over and a great
variety of poorer and less exact historical kinds that are nonetheless not
nominal but real.


The category of substances, as I have defined it, is at root an epistemo-
logical category. As such, it cuts straight across many more familiar dis-
tinctions in ontology. What makes a substance a substance is that it can
be appropriated by cognition for the grounded, not accidental, running
of inductions, or projecting of invariants. This will be possible in differ-

projectible, because if one is copied the other may be too. So, for instance, while there is
no law (so far as I know) connecting having a chitinous exoskeleton and having more than
four legs, it is reasonable to assume that something with a chitinous exoskeleton has more
than four legs because something with the exoskeleton is a copy of something else with
an exoskeleton that had more than four legs” [private correspondence].

ent cases for very different reasons, due to very different sorts of causes,
which is, of course, exactly what interests me about substances. It is their
variety, considered from other ontological perspectives, that makes it easy
to overlook their similarity relative to the projects of cognition. I have
illustrated the category of substance by reference to individuals, stuffs
and certain kinds whose members are ordinary physical individuals. But
other ontological types can be substances too. Beethoven™s Fifth Sym-
phony has many properties that are unlikely to vary from performance
to performance. You can recognize it and know what is coming next.
This is also true of tellings of The Three Bears. Places have properties
many of which remain the same over time. Dinner time and siesta time
have pretty definite properties, in many cultures.War among humans has
certain properties that seem to remain pretty much the same over the
ages. Western industrial economies can be studied as a real kind.
There is not one set of ontological “elements,” one unique way of
carving the ontology of the world, but a variety of crisscrossing over-
lapping equally basic patterns to be discovered there. Cubes are things
one can learn to recognize and learn a number of stable things about
such as how they fit together, how they balance, that their sides, angles
and diagonals are equal, and so forth. In their commentary on Millikan
(1998a), Cangelosi and Parisi (1998) remark (correcting me) that white
thing is something one can learn about.White things, they said, get dirty
easily and, I now add, show up easily in dim light, stay cool in sunlight,
but also tend to blind us, and so forth. Understood as substances, how-
ever, I think that these entities are most naturally and also most cor-
rectly named with simple nouns: “Cubes don™t stand on edge easily,”
“White stays cool in sunlight,” “squares have equal diagonals” and
so forth. This reflects the fact that qua naming substances, the
terms “cube,” “white,” and “square” express subject-term thoughts. As
substances, white and square are not predicates, not properties; they
have properties. The same thing can be a property relative to certain
substances and also a substance relative to certain properties.7 Which
way a thinker is understanding such an entity is generally expressed
in the grammar.

7 For more on this theme, see Millikan (1984), Chapters 15“17. There I claim, for example,
that unlike substances, properties are, as such, members of contrary spaces.These are groups
whose members oppose one another, by natural necessity, on the ground of certain kinds
of substances.

Just as properties do not have to have natural demarcation lines be-
tween them in order to be real, there are substances that have no nat-
ural boundaries along certain dimensions. Water shades into mud on
one side and into lemonade and then lemon juice on another. Sub-
stances of this sort are organized around paradigms, or around peak
points, or gradient shifts, at which causally intertwined properties are ei-
ther historically or ahistorically determined to be collected together.
Other cases often diverge from the paradigms along several dimensions.
Closer approximation to the paradigm essences or paradigm historical
causes linking these cases together yields closer approximation to the
other of typical properties of the substance as well.


I can observe today that Xavier has blue eyes and knows Greek, and un-
less Xavier is very unlucky, this will hold true when I meet him to-
morrow. But if Xavier is sitting or angry or playing tennis when I meet
him, this probably will not be true tomorrow. Similarly, if I observe the
approximate adult size, preferred diet, variety and placement of internal
and external organs (two eyes, two kidneys, one heart on the left) and
general physiology of one member of the species Felis domesticus, all of
these observations will probably yield correct predictions about the next
member of Felis domesticus. But if my observations concern color, cer-
tain kinds of behavior patterns, and the pattern of torn ears, they will
be unlikely to carry over to the next cat I meet. If they do, it will be a
matter of accident. Again, if I have determined the color, boiling point,
specific gravity, volatility, and chemical combining properties of diethyl
ether on one pure sample, then I have determined the color, boiling
point, specific gravity, volatility, and chemical combining properties of
diethyl ether, period. If the experiments need replication, this is not be-
cause other samples of diethyl ether might have a different color, boil-
ing point, and so forth, but because I may have made a mistake in mea-
surement or analysis. But I cannot in this way determine the shape,
volume, or purity of diethyl ether. These are not properties that gener-
alize from one meeting to the next.
Now about diethyl ether you probably take me to be right, not be-
cause you know that the above is true of diethyl ether specifically.
Rather, you know it is true of chemical compounds generally. You
know that chemical compounds do not vary with respect to color, boil-

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