The actions of the mathematics teacher are bound up in ethical decisions that impact the learner and teacher, both within and external to the formal school curriculum. This paper argues that the principles of a radical constructivist theory of knowing underlie a model for an ethics of liberation. A learner’s active construction of their experiential reality, including the construction of the independent existence of the other and resulting social implications, frame guidelines for actions that are liberatory. To demonstrate this point, the paper develops the ideas of responsibility of the self, unique directions of learning, and socially-generated disequilibrium. When teachers and their students act according to such guidelines, they are freed to know mathematics and hence themselves in ways that allow them to work toward social justice and democratic ideals.