Mathematical realism asserts that mathematical objects exist in the abstract world, and that a mathematical sentence is true or false in virtue of the way the abstract world is. In contrast, mathematical fictionalism claims that the abstract world does not exist, but mathematical sentences purport to be about the abstract world, so they are all false. I defend a new position which I call mathematical inferentialism. It holds that the abstract world does not exist, and that a mathematical sentence is true if and only if all of its concrete consequences are true. It has several advantages over mathematical realism and fictionalism.