I'm almost done with the first volume in Copleston's History of Philosophy, the one on Greece and Rome. Copleston pauses, before the index appears, to look back at the world of ancient philosophy, and he mentions the issue of "the one and the many" as a key theme.
In the middle ages it became the "problem of universals".
The problem, the issue, is that we deal in particulars, but we perceive generalities. How can that be? You can say that we are pattern-recognizing beings. As are lots of animals. But we carry it to extremes. And we've even gotten to the point where we can build machines that mimic, in some ways, our ability to pick out patterns.
So we recognize them, but why are they there to be recognized? Why is reality such that patterns always emerge when you look for them? Granted, it's hard to imagine an unpatterned reality - a reality without rules, without regularity, a reality consisting of nothing but chaos.
In a reality without rules
The wisest would be fools.