Science usually begins after a certain assumption has already been granted: that nature is stable enough to be studied. An astronomer who measures the motion of a planet expects the same physical relationships to hold elsewhere. A chemist assumes that the behavior of oxygen today will still tell us something about oxygen tomorrow. Without some degree of regularity, scientific explanation would lose its footing because observations from the past would have little authority over what happens next. David Hume saw the difficulty in this long before modern physics. We can observe that certain events have followed others repeatedly, but observation alone never gives us a logical guarantee that the pattern must continue. The fact that nature has behaved consistently does not itself explain why consistency should persist. That leaves a question underneath the ordinary work of science: when we say that the universe follows laws, what exactly are we claiming exists? Are Laws Actually in the Univ...