What are the theoretical foundations of mathematics education? Recently disciplines other than mathematics and psychology have grown in importance, including philosophy. But which branch of philosophy is the most fundamental for mathematics education? In this article, I consider the claims of five branches of philosophy to be our "first philosophy." I examine the contributions and priority claims of Critical Theory, philosophy of mathematics, epistemology, ontology and ethics and conclude that ethics plays the role of first philosophy for mathematics education. This is because, following Levinas, ethics precedes all theorizing or conscious reflection about any human activities including the teaching and learning of mathematics. This has implications for such things as the acceptability of radical constructivism.