Try reading up on Hilbert's second problem. Wikipedia does a good job [1]. Then read about Peano axioms [2]. Finally try to understand what is meant by the term "consistency" [3]

1. http://en.wikipedia.org/wiki/Hilbert%27s_second_problem

2. http://en.wikipedia.org/wiki/Peano_axioms

3. http://en.wikipedia.org/wiki/Consistency#Consistency_and_com...