"What is Mathematics, Really?" by Reuben Hersh, in Google Books.
"What is Mathematics? An Elementary Approach to Ideas and Methods", Richard Courant, Herbert Robbins, in Google Books.
Mathematical Intuition (Poincaré, Polya, Dewey) by Reuben Hersh, University of New Mexico, TMME, vol8, nos.1&2, p .35
The Number Sense : How the Mind Creates Mathematics, by Stanislas Dehaene, Google Books.
The Psychology of Number and its Applications to Methods of Teaching Arithmetic by James A. McLellan; John Dewey, at www.openlibrary.org.
How Math Can Be Applied To So Many Different Fields, Mathematics and Quantitative Reasoning web site, B. Harford