“The improved calculation of Mercury’s orbit was, in fact, a crucial first test of Einstein’s new theory. What moral to draw, then, of the non-existence of Vulcan and the subsequent triumph of general relativity? At the least this: science is unique among human ways of knowing because it is self-correcting. Every claim is provisional, which is to say each is incomplete in some small or, occasionally, truly consequential way. But in the midst of the fray, it is impossible to be sure what any gap between knowledge and nature might mean.”