Aristotle makes the claim that, "the fact that a thing is itself" allows for "a fixed constant nature of sensible things"
Does it mean:
- sensible things look like fixed and constant, or
- sensible things are fixed and constant?
What does nature mean?
Literally, that the nature of "things", at least, sensible things, is fixed and constant. A thing is itself, reflexively, and we can proceed from that point without having to reexamine the nature of it as we draw inferences from it.
Here's Wikipedia's take: http://en.wikipedia.org/wiki/Law_of_identity
Think about Descartes' famous "I think, therefore I am". What he's saying there is, because I am able to contemplate my own existence, I can reasonably assume my own existence. Without that assumption, nothing real can be inferred, and it's all just college dorm room B.S. Instead, a thing is itself, and that allows for a fixed, constant nature of (at least) sensible things.
Lacking further context, I would interpret this as: All sensible things have a nature (that is to say, a set of properties or qualities, or a 'character' or 'basic constitution'); that nature is fixed and constant.