The initial situation of knowledge is that when we look at this and try to describe it, it is impossible for us to mention anything that could just as easily have belonged to a non-this. The sentence just said is not an exception to itself: the very words “this” and its negation are contained in the claim. We do have proper names, but any attempt to understand what they are naming collapses into the same problem.
Plato’s response to this is to say that the general names simply are what exists – whatever might fall under a proper name is not a subsistent reality or “thing in itself” but only, at best, a thing by another. This doctrine probably faces insurmountable problems in the face of the cogito, i.e. in face of my awareness that James Chastek exists. Aristotle’s response is unclear and ended up with a wide divergence of interpretation. All sides begin with an agreement that understanding abstracts from primary substance and that primary substance was the fundamental reality, but there was a subsequent disagreement over the reality of the thing abstracted. Just what the correct interpretation is will never be known, but at the end of the day we are left with some version of the claim that what is fundamentally real is not intelligible to us.
And so we have the original problem of knowledge. Any theory of knowledge has to account for how I knowl the real, but Plato’s account cannot explain the I and Aristotle’s cannot explain the real. We either get an explanation of the subject or the predicate in I know the real, but not both.
As said, this is an initial problem and not a final one: the whole point is what we say next.