Numbers exist, that is clear to me, but is there any logical correlation between numbers and sensory experience? This question came while I was reading Einstein's comments on Bertrand Russell's theory of knowledge. Einstein says that it is clear that the series of integers is a creation of the human spirit and that there is no way we could crystallize this concept (of integers) through sensory experience. Why is that? I can feel if one or two things are touching me and I can see if there are one or two sheep in front of me. Is he talking about the number without any kind of object attached to it (one is different than one sheep)? Can this be explained even though I didn't read Russell's theory of knowledge?