Nominalism, in turn, faces the difficulty of having to explain the successful use of mathematics in scientific theorizing. Since, according to the nominalist, mathematical objects do not exist—or, at least, are not taken to exist—it becomes unclear how referring to such entities can contribute to the empirical success of scientific theories. In particular, if it turns out that reference to mathematical entities is indeed indispensable to our best theories of the world, how can the nominalist deny the existence of such entities? As we will see below, several nominalist views in the philosophy of mathematics have emerged in response to the challenge raised by considerations based on the indispensability of mathematics.