zlacker

The part about defining identity is, I think, referring to the fallacy of seeing someone in a context, and then thinking that they "are" that. Like meeting someone who is angry and storing them in your memory bank as "that angry person," as if they were angry all the time (they may indeed be, but you don't know). Add to that, the errors inherent in the act of perception: Are they really angry or did you misperceive it that way?

In your example, the co-worker may be "your disagreeable co-worker" (definition), or they might simply be "your agreeable co-worker" who momentarily disagrees with you or your ideas.

If your opinion about someone is right enough, often enough, that it serves as a workable summary of that person, that (ideally) a lot of people agree on, who have no vested interest in agreeing, then you could start to be objectively convinced there was no difference between your opinion of that person, and the definition of that person. But no simple definition of a person is ever going to capture the whole story - people are too complex.