Summary
Referring back to his logical work in the Categories, Aristotle
opens book Zeta by asserting that substance is the primary category
of being. Instead of considering what being is, we can consider
what substance is.
Aristotle first rejects the idea that substance is the
ultimate substrate of a thing, that which remains when all its accidental
properties are stripped away. For example, a dog is more fundamental
than the color brown or the property of hairiness that are associated
with it. However, if we strip away all the properties that a dog
possesses, we wind up with a substrate with no properties of its
own. Since this substrate has no properties, we can say nothing
about it, so this substrate cannot be substance.
Instead, Aristotle suggests that we consider substance
as essence and concludes that substances are species. The essence
of a thing is that which makes it that thing. For example, being
rational is an essential property of being human, because a human
without rationality ceases to be human, but being musical is not
an essential property of being human, because a human without musical
skill is still human. Individual people, or dogs, or tables, contain
a mixture of essential and inessential properties. Species, on the
other hand—for instance, people in general, dogs in general, or
tables in general—contain only essential properties.
A substance can be given a definition that does not presuppose the
existence of anything else. A snub, for example, is not a substance,
because we would define a snub as “a concave nose,” so our definition
of snub presupposes the existence of noses. A proper definition
of a thing will list only its essential properties, and Aristotle asserts
that only substances have essential properties or definitions. A
snub nose, by contrast, has only accidental properties—properties
like redness or largeness that may hold of some snubs but not of all—and per
se properties—properties like concavity, which necessarily
holds of all snubs but which is not essential.
Physical objects are composites of form and matter, and
Aristotle identifies substance with form. The matter of an object
is the stuff that makes it up, whereas the form is the shape that
stuff takes. For example, the matter in a bronze sphere is the bronze
itself, and the form is the spherical shape. Aristotle argues that
form is primary because form is what gives each thing its distinctive
nature.
Aristotle has argued that the definitions of substances
cannot presuppose the existence of anything else, which raises the
question of how there can be a definition that does not presuppose
the existence of anything else. Presumably, a definition divides
a whole into its constituent parts—for example, a human is defined
as a rational animal—which suggests that a substance must in some
way presuppose the existence of its constituent parts. Aristotle
distinguishes between those cases where the parts of an object or
definition are prior to the whole and those cases where the whole
is prior to the parts. For example, we cannot understand the parts
of a circle without first understanding the concept of circle as
a whole; on the other hand, we cannot understand the whole of a
syllable before we understand the letters that constitute its parts.
Aristotle argues that, in the case of substance, the whole is prior
to the parts. He has earlier associated substance with form and
suggests that we cannot make sense of matter before we can conceive
of its form. To say a substance can be divided by its definition
is like saying a physical object can be divided into form and matter:
this conceptual distinction is possible, but form and matter constitute
an indivisible whole, and neither can exist without the other. Similarly,
the parts of a definition of a substance are conceptually distinct,
but they can only exist when they are joined in a substance.
