There is however in Plato and among philosophers of certain other schools a very different doctrine, to the effect that there is nothing worthy to be called ‘knowledge’ to be derived from the senses, and that the only real knowledge has to do with concepts. … In this view, ‘2 + 2 = 4’ is genuine knowledge, but such a statement as ‘snow is white’ is so full of ambiguity and uncertainty that it cannot find a place in the philosopher’s corpus of truths.

Bertrand Russell, History of Western Philosop…, loc. 674