Quantum mechanics is the second great watershed of twentieth century physics, and in some ways it involved an even more radical break with galilean orthodoxy than relativity did. But the nature of this second rift is somewhat obscure. That is partly because quantum mechanics has such a radical and unfamiliar form, but it is also partly because the mathematical formalism of quantum mechanics preceded its interpretation to an unusual degree. Physicists were in possession of the mathematical expression of quantum mechanics before they had a good sense of what it said about the world. In fact, to some degree it is still far from clear what that spectacularly successful formalism says about the world.