Let R be the whole of reality and let x be an entity within R. Is it, in order to truly and totally know x, necessary to know R? If so, it would follow that complete knowledge of something is impossible, since no human knows R.
At least on theism the answer seems to be 'yes'. For one would have to know God fully - and thereby plausibly R fully - in order to know the complete nature of things.
In a perhaps trivial sense the answer might be always 'yes'. For if one doesn't know R, one doesn't know how x and R are related to each other, and thus one doesn't know x fully if we take it that knowledge of x's relations is part of knowing x.