Iam using Spivak these days for learning calculus. In page 22, I have difficulty understanding. He speaks about natural numbers. Do natural numbers always start with 1? He talks about the definition of a set of natural numbers as having 1. Always 1 in set. 2. If k is present, k+1is also present. Then cant I say, since 1 were present, 0 must also be present. Else, the second rule is not satisfied. Am I missing something?