Aristotle - The Organon - index for CATEGORIAE Book 1 Part 5

Substance

Paragraph 1	Substance, in the truest and primary and most definite sense of the word, is that which is neither predicable of a subject nor present in a subject;
Paragraph 2	It is plain from what has been said that both the name and the definition of the predicate must be predicable of the subject.
Paragraph 3	With regard, on the other hand, to those things which are present in a subject, it is generally the case that neither their name nor their definition is predicable of that in which they are present.
Paragraph 4	Everything except primary substances is either predicable of a primary substance or present in a primary substance.
Paragraph 5	Of secondary substances, the species is more truly substance than the genus, being more nearly related to primary substance.
Paragraph 6	Moreover, primary substances are most properly called substances in virtue of the fact that they are the entities which underlie everything else, and that everything else is either predicated of them or present in them.
Paragraph 7	Of species themselves, except in the case of such as are genera, no one is more truly substance than another.
Paragraph 8	It is, then, with good reason that of all that remains, when we exclude primary substances, we concede to species and genera alone the name 'secondary substance', for these alone of all the predicates convey a knowledge of primary substance.
Paragraph 9	Further, primary substances are most properly so called, because they underlie and are the subjects of everything else.
Paragraph 10	It is a common characteristic of all substance that it is never present in a subject.
Paragraph 11	Yet this is not peculiar to substance, for it is also the case that differentiae cannot be present in subjects.
Paragraph 12	The fact that the parts of substances appear to be present in the whole, as in a subject, should not make us apprehensive lest we should have to admit that such parts are not substances:
Paragraph 13	It is the mark of substances and of differentiae that, in all propositions of which they form the predicate, they are predicated univocally.
Paragraph 14	All substance appears to signify that which is individual.
Paragraph 15	Yet species and genus do not merely indicate quality, like the term 'white';
Paragraph 16	Another mark of substance is that it has no contrary.
Paragraph 17	Substance, again, does not appear to admit of variation of degree.
Paragraph 18	The most distinctive mark of substance appears to be that, while remaining numerically one and the same, it is capable of admitting contrary qualities.
Paragraph 19	If, then, a man should make this exception and contend that statements and opinions are capable of admitting contrary qualities, his contention is unsound.
Paragraph 20	But it is by reason of the modification which takes place within the substance itself that a substance is said to be capable of admitting contrary qualities;
Paragraph 21	To sum up, it is a distinctive mark of substance, that, while remaining numerically one and the same, it is capable of admitting contrary qualities, the modification taking place through a change in the substance itself.
Paragraph 22	Let these remarks suffice on the subject of substance.