Problem: What is it that Ernst von Glasersfeld brought to mathematics education with radical constructivism? Method: Key ideas in the author’s early thinking are related to ideas that are central in constructivism, with the aim of showing their importance in math education. Results: The author’s initial thinking about constructivism began with Toulmin’s view of thinking as evolving. Ernst showed how Piaget’s genetic epistemology implied an epistemology that was not about ontology. Continuing with an analysis of the way radical and trivial (...) constructivism were received by the mathematics education community, implications of Ernst’s ideas are considered. Implications: These include the need to consider major changes in ways content is introduced to children, to consider carefully the language used to describe children’s emerging mathematical ideas, and to consider new conjectures and also how we think about the foundations of mathematics. Ultimately the value of RC is the way it reinspires belief in the possibility and importance of human growth. (shrink)