“Things are said to ‘be’ (1) in an accidental sense, (2) by their own nature.
“(1) In an accidental sense, e.g. we say ‘the righteous doer is musical’, and ‘the man is musical’, and ‘the musician is a man’, just as we say ‘the musician builds’, because the builder happens to be musical or the musician to be a builder; for here ‘one thing is another’ means ‘one is an accident of another’. So in the cases we have mentioned; for when we say ‘the man is musical’ and ‘the musician is a man’, or ‘he who is pale is musical’ or ‘the musician is pale’, the last two mean that both attributes are accidents of the same thing; the first that the attribute is an accident of that which is, while ‘the musical is a man’ means that ‘musical’ is an accident of a man. (In this sense, too, the not-pale is said to be, because that of which it is an accident is.) Thus when one thing is said in an accidental sense to be another, this is either because both belong to the same thing, and this is, or because that to which the attribute belongs is, or because the subject which has as an attribute that of which it is itself predicated, itself is.
“(2) The kinds of essential being are precisely those that are indicated by the figures of predication; for the senses of ‘being’ are just as many as these figures. Since, then, some predicates indicate what the subject is, others its quality, others quantity, others relation, others activity or passivity, others its ‘where’, others its ‘when’, ‘being’ has a meaning answering to each of these. For there is no difference between ‘the man is recovering’ and ‘the man recovers’, nor between ‘the man is walking or cutting’ and ‘the man walks’ or ‘cuts’; and similarly in all other cases.
“(3) Again, ‘being’ and ‘is’ mean that a statement is true, ‘not being’ that it is not true but falses-and this alike in the case of affirmation and of negation; e.g. ‘Socrates is musical’ means that this is true, or ‘Socrates is not-pale’ means that this is true; but ‘the diagonal of the square is not commensurate with the side’ means that it is false to say it is.
“(4) Again, ‘being’ and ‘that which is’ mean that some of the things we have mentioned ‘are’ potentially, others in complete reality. For we say both of that which sees potentially and of that which sees actually, that it is ‘seeing’, and both of that which can actualize its knowledge and of that which is actualizing it, that it knows, and both of that to which rest is already present and of that which can rest, that it rests. And similarly in the case of substances; we say the Hermes is in the stone, and the half of the line is in the line, and we say of that which is not yet ripe that it is corn. When a thing is potential and when it is not yet potential must be explained elsewhere.
“We call ‘substance’ (1) the simple bodies, i.e. earth and fire and water and everything of the sort, and in general bodies and the things composed of them, both animals and divine beings, and the parts of these. All these are called substance because they are not predicated of a subject but everything else is predicated of them.-(2) That which, being present in such things as are not predicated of a subject, is the cause of their being, as the soul is of the being of an animal.-(3) The parts which are present in such things, limiting them and marking them as individuals, and by whose destruction the whole is destroyed, as the body is by the destruction of the plane, as some say, and the plane by the destruction of the line; and in general number is thought by some to be of this nature; for if it is destroyed, they say, nothing exists, and it limits all things.-(4) The essence, the formula of which is a definition, is also called the substance of each thing.
“It follows, then, that ‘substance’ has two senses, (A) ultimate substratum, which is no longer predicated of anything else, and (B) that which, being a ‘this’, is also separable and of this nature is the shape or form of each thing.
Metaphysics by Aristotle – Book V