(a) Using a predicate which is said of but not in the subject. 'Man' is said of the individual man, as in 'Socrates is a man.' But unlike the color white, man is not in Socrates.
(b) Using a predicate which in the subject (though it does not exist separately from the subject, as Plato had held) but not said of it. Socrates' knowledge of grammar is in Socrates's soul. But it is ungrammatical to say that Socrates is knowledge of grammar.
(c) Using a predicate which both is said of a subject and in a subject. Knowledge of grammar is knowledge, hence 'knowledge' is said of something, and knowledge is in Socrates' soul.
(d) By referring to a subject itself, as a thing which is "individual and numerically one" and not said of or in a subject. But note that an individual may be in a subject. Socrates' own knowledge of grammar is an individual, but it is in Socrates.
We can classify things by genus and specific difference. For example, in the Euthyphro, Socrates stated the piety is a part of justice which concerns the care of the gods. Justice is the genus (compare the word 'generic') and concering care of the gods is the difference which separates it from other kinds of justice.
Aristotle enunciates a principle. That if two genera are different and not subordinated to each other, then the specific differences are different in kind. Animal is different form knowledge; further, animal is not knowledge and knowledge is not animal, so they are not subordinate. One way of differentiating animals is by whether they have wings, and one way of differentiating knowledge is by whether it is of grammar. So being winged and being grammatical are different in kind.
What makes primary substance most real is that everything else depends on it. For something to be in a subject, there must be a subject for it to be in. There would be no knowledge of grammar if there were no individuals to have the knowledge. For something to be said of a subject, again there must be a subject of which it can be said. "So, if primary substances did not exist it would be impossible for any of the other things to exist."
The species is more a substance than the genus, because closer to primary substance. It is more informative to say of Socrates that he is man than that he is animal.
A substance is not in a subject. Primary substance is neither in nor said of one, whereas secondary substance, although it is said of man, it is not in man. [Again, this is in opposition to Plato.]
Substance signifies a certain "this," clearly in the case of primary substances (individual and numerically one). Once more in opposition to Plato, who thought of forms as individuals, secondary substances don't really signify a this, but a qualification. For example, the predicate 'white' signifies a qualification of substance.
Nothing is contrary to substance. It does not admit a more or a less.
The most distinctive characteristic of substance is that it is numerically one and the same, and able to receive contraries. (Color does not receive black and white, the same action is not both bad and good. But a man can be pale at one time and dark at another; this holds as well for hot and cold, good and bad.)
Introduction to Philosophy Home Page