The Complete Aristotle (eng.)

that are in the full sense (for the thought
attaches or removes either the subject’s ‘what’ or its having a
certain quality or quantity or something else), that which is
accidentally and that which is in the sense of being true must be
dismissed. For the cause of the former is indeterminate, and that
of the latter is some affection of the thought, and both are
related to the remaining genus of being, and do not indicate the
existence of any separate class of being. Therefore let these be
dismissed, and let us consider the causes and the principles of
being itself, qua being. (It was clear in our discussion of the
various meanings of terms, that ‘being’ has several meanings.)

Book VII
    Translated by W. D. Ross
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1
    There are several senses in which a thing may be said to ‘be’,
as we pointed out previously in our book on the various senses of
words;’ for in one sense the ‘being’ meant is ‘what a thing is’ or
a ‘this’, and in another sense it means a quality or quantity or
one of the other things that are predicated as these are. While
‘being’ has all these senses, obviously that which ‘is’ primarily
is the ‘what’, which indicates the substance of the thing. For when
we say of what quality a thing is, we say that it is good or bad,
not that it is three cubits long or that it is a man; but when we
say what it is, we do not say ‘white’ or ‘hot’ or ‘three cubits
long’, but ‘a man’ or ‘a ‘god’. And all other things are said to be
because they are, some of them, quantities of that which is in this
primary sense, others qualities of it, others affections of it, and
others some other determination of it. And so one might even raise
the question whether the words ‘to walk’, ‘to be healthy’, ‘to sit’
imply that each of these things is existent, and similarly in any
other case of this sort; for none of them is either self-subsistent
or capable of being separated from substance, but rather, if
anything, it is that which walks or sits or is healthy that is an
existent thing. Now these are seen to be more real because there is
something definite which underlies them (i.e. the substance or
individual), which is implied in such a predicate; for we never use
the word ‘good’ or ‘sitting’ without implying this. Clearly then it
is in virtue of this category that each of the others also is.
Therefore that which is primarily, i.e. not in a qualified sense
but without qualification, must be substance.
    Now there are several senses in which a thing is said to be
first; yet substance is first in every sense-(1) in definition, (2)
in order of knowledge, (3) in time. For (3) of the other categories
none can exist independently, but only substance. And (1) in
definition also this is first; for in the definition of each term
the definition of its substance must be present. And (2) we think
we know each thing most fully, when we know what it is, e.g. what
man is or what fire is, rather than when we know its quality, its
quantity, or its place; since we know each of these predicates
also, only when we know what the quantity or the quality is.
    And indeed the question which was raised of old and is raised
now and always, and is always the subject of doubt, viz. what being
is, is just the question, what is substance? For it is this that
some assert to be one, others more than one, and that some assert
to be limited in number, others unlimited. And so we also must
consider chiefly and primarily and almost exclusively what that is
which is in this sense.
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2
    Substance is thought to belong most obviously to bodies; and so
we say that not only animals and plants and their parts are
substances, but also natural bodies such as fire and water and
earth and everything of the sort, and all things that are either
parts of these or composed of these (either of parts or of the
whole bodies), e.g. the physical universe and its parts, stars and
moon and sun. But whether these alone are substances, or there are
also others, or only some of these, or others as well, or none of
these but only some other things, are substances, must be
considered. Some think the limits of body, i.e. surface, line,
point, and unit, are substances, and more so than body or the
solid.
    Further, some do not think there is anything substantial besides
sensible things, but others think there are eternal