Everettian quantum theory describes physical reality as a vast multiverse, which is almost entirely imperceptible to us (“almost” because we perceive a famous but infinitesimal slice called the universe). Many people reject this theory, either because it asserts the existence of imperceptible entities, or because it seems to violate Occam’s razor, which advises us not to multiply beyond necessity the entities referred to by our theories. The first criticism is based on a false philosophy of science known as empiricism, which holds that observations are irreducible primitives in science, and that all scientific knowledge derives from experience. If empiricism were true, then humans would possess no knowledge, for example, of events that occurred before humans existed to make observations; thus, the fact that we possess such knowledge (e.g., of long-extinct species and of the formation of the moon) straightforwardly refutes empiricism. The second criticism, by invoking the principle of Occam’s razor, suffers from two major flaws that characterize the principle itself: first, Occam’s razor suggests the need for a criterion of “necessity”, yet Occam’s razor specifies no such criterion; second, Occam’s razor defines the parsimony of a theory in terms of how many entities the theory refers to, but parsimony is about the structure of the theory, not the structure of the world that the theory describes. A single rationalist maxim undermines both of these criticisms of Everettian quantum theory: namely, that all theories should be judged by their explanatory power. Compared to all rival explanations of quantum phenomena such as interference, Everettian quantum theory stands as the only realistic and parsimonious explanation. For as long as we lack any better explanation of quantum phenomena, the explainer must conclude—despite all protestations from common sense—that we live inside of a dizzyingly vast, mostly hidden, quantum multiverse.