Introuction to Arithmetic was written by Nicomachus almost two thousand years ago, and contains both philosophical prose and very basic mathematical ideas. Nicomachus refers to Plato quite often, and wrote about how philosophy can only be possible if one knows enough maths. This is the only complete book of his that survived to our day. Nicomachus describes how natural numbers and basic mathematical ideas are eternal and unchanging, and in an incorporeal realm.