It's interesting, but if you don't have time to read this entire Economist article, here is a quick summary:
Newborn babies can tell the difference between 2 and 3. Apparently, numbers and counting are built into our brains. Even Aboriginal people who don't have words for numbers beyond 2. Also, animals can count. Plato and Piaget were wrong, but I guess they did the best they could with the data they had.
Easy as 1,2,3 [economist via, uhh, I forget]