Quantum theory is '... the blackest of black-box theories: a marvellous predictor but an incompetent explainer"(Fine 1982) I will discuss why I think that the best way to understand quantum systems is to re-interpret it using topos theory. I will consider Galilean as well as special relativistic massive particle theories and try to explain how some of the paradoxes of quantum theories can be understood when quantum real numbers in a topos are the ontological values taken by physical quantities.