What's the meaning of “in virtue of itself”?

Question

Therefore, as Aristotle expressed it, things are what they are only relative to other things, and nothing is what it is simply in virtue of itself

Answer 1

The New Oxford American Dictionary has

by (or in) virtue of: because or as a result of

which here means “by being itself”, “by existing” or, at Caleb put it, “on its own”.

Answer 2

What you are asking about concerns philosophical relativism, of which Aristotle was an early critic. He felt that if such a concept were applied only to appearances, contradictions would occur if those apparent qualities were applied indiscriminately to all things. The solution to this paradox was his notion of essentialism:

In simple terms, essentialism is a generalization stating that certain properties possessed by a group (e.g. people, things, ideas) are universal, and not dependent on context.

The fragment that you cite in your example is actually Aristotle framing his opponents' argument so that he might reject it.

Answer 3

This is a strange way to make a statement and not in common usage, but in this case you could replace "simply in virtue of itself" with "on its own". Basically the statement is saying that things do not have any intrinsic identity, they are defined by their relation to other things.