The scientific community takes for granted a view of science that may be called standard empiricism. This holds that the basic intellectual aim of science is truth, nothing being presupposed about the truth, the basic method being to assess theories with respect to evidence. A basic tenet of the view is that science must not accept any thesis about the world as a part of scientific knowledge independent of evidence, let alone in violation of evidence. But physics only accepts unified theories, and persistently rejects infinitely many ad hoc rivals that fit the phenomena even better. In persistently rejecting these infinitely many empirically more successful rival theories, physics thereby makes a substantial assumption about the universe – it is such that all ad hoc theories are false – an assumption that is accepted implicitly independently of evidence, even in a sense against the evidence. That contradicts standard empiricism. The scientific community needs to adopt a new conception of science that represents the assumption of physics as a hierarchy of assumptions, thus facilitating the improvement of the assumption that is made, as science proceeds.