Thinking from a fundamental or foundational ontology perspective ... I want to read what philosophers have written about a concept which in my own mind I have been calling "variation".
I am thinking about physical Reality, that is, at the most irreducible level that I can conceive of Reality. So, I start with one particular existent thing (aka "particle," "atom," or "monad" maybe) and think about concepts that are an outflow of the existence of one particular thing.
After that I move on to consider the possibility of two particular physically existent things. As soon as I think about two particular physical things, some other concepts come to mind ... like quantity, relations, finiteness/limits, and "variation". Reasoning about this physical "variation" might be framed by some proposition like "Thing1 is different from Thing2," or restated a little more precisely, "Thing1 has at least one different physical property from Thing2."
It seems that whether or not variation would exist in Reality is fundamental, and how we understand this "variation". Here I am NOT talking about trying to delineate what specific variation may or may not exist between Thing1 and Thing2, so that we would ascribe a specific physical property, but just trying to think about physical variation itself and trying to precisely define it. I also am not talking about the knowledge of variation by comparing two things, but am talking about the fact of physical variation between things in Reality. Smart people must have written about this already.
Please (1) tell me the current philosophy jargon for what I have been calling "variation", (2) point me toward important documents where philosophers have written coherently on this subject, (3) tell me if those philosophers used words other than what you identified as current jargon to talk about this subject, (4) give me any advice you have about reasoning on this subject.
Thank you for your time.