Mathematics is often successfully used in scientific theories. How can such a success be explained? Platonists allegedly have an answer to this problem. Given that mathematical objects exist and are successfully referred to by our scientific theories, it is not surprising that such theories are successful. Reference to mathematical objects is just part of the reference to those entities that are indispensable to our best theories of the world. This frames the problem of the application of mathematics in terms of the indispensability argument.