Internet Encyclopedia of Philosophy: "Henri Poincare" by Mauro Murzi. Includes a discussion of Poincaré's critique of the Peano's axioms. First-order arithmetic, a chapter of a book on the incompleteness theorems by Karl Podnieks. What are numbers, and what is their meaning?: Dedekind commentary on Dedekind's work, Stanley N. Burris, 2001.