Accounting class, uncertainty.

I sit in this class learning about numbers and journals that are rarely used, but commonly understood, for what? While drifting off, a common action I take part in on Tuesday and Thursday afternoons, I began thinking about numbers. How do we recognize that 1 has a value of 1 and 2 a value of 2? Pythagorean realized that the world is Mathematical, believed that everything could be determined with exact certainty. But how exactly can we obtain this certainty? Especially when in doing math someone can make a mistake and write a 2 instead of a 1. If the 2 was written with certainty, doesn't it defeat the purpose of having the Pythagorean observation of Mathematics? If 2 was in fact the wrong number, then how can we be certain it was right when we wrote it?