Aristotle holds that we only have scientific knowledge of what cannot be otherwise. This may seem to imply that we only have scientific knowledge of changeless mathematical truths and other products of a priori reflection. Yet Aristotle is a pioneer of natural science, and exhorts us to study the natural world, which he himself characterizes as a realm of contingency, exception and chance. This dissertation asks why Aristotle holds the view that we only know what cannot be otherwise and whether he is able to reconcile this view with his engagement in and esteem for natural science, especially the study of animals. Aristotle holds that we only have scientific knowledge of necessities, I argue, as a way to reconcile his view that knowledge requires persisting agreement with the world and his view that scientific knowledge remains stable over time. Properly understood, Aristotle’s claim does not pose a threat to the possibility of natural science but rather is at the heart of an attempt to explain how the study of nature is possible. We can have scientific knowledge about changeable and capricious things because the content of our knowledge strictly extends only to those facts about them that remain perpetually true on account of their essences.