Aristotle has just shown that substance is a kind of unity. He continues the Categories by showing that substance has no contraries.
Another characteristic of substances is that there is nothing contrary to them. For what would be contrary to a primary substance? For example, there is nothing contrary to an individual man, nor yet is there anything contrary to man or to animal. This, however, is not peculiar to substance but holds of many other things also, for example, of quantity. For there is nothing contrary to four-foot or to ten or to anything of this kind—unless someone were to say that many is contrary to few or large to small; but still there is nothing contrary to any definite quantity.
Nothing is contrary to substance. This is true of both primary and secondary substances. A contrary is the opposite of something. But nothing is the opposite of an individual person, humanity or rocks. But lacking contraries is not something unique to substances. Quantities also lack contraries. If we pick any number, there is no number contrary to it. We might say that the opposite of large is small, but actual numbers don’t have contraries.
It is very important to note that Aristotle clearly says that this is not a unique feature of substances. This distinction is different than the one made later about gaining contrary qualities.
Next, Aristotle shows that substances cannot be measured.