This paper examines the role of reason in Shepherd's account of acquiring knowledge of the external world via first principles. Reason is important, but does not have a foundational role. Certain principles enable us to draw the required inferences for acquiring knowledge of the external world. These principles are basic, foundational and, more importantly, self-evident and thus justified in other ways than by demonstration. Justificatory demonstrations of these principles are neither required, nor possible. By drawing on textual and contextual evidence, I will show that Shepherd should have said that we know the first principles of any science, in general, and that "everything which begins to exist must have a cause", in particular, via intuition, not via reason. Reasoning about such principles can help their self-evidence shine through in certain cases; their justification, and our being justified in believing them, does not come from this reasoning, however.