I argue for Aristotle’s greatness as a philosopher by highlighting the systematic nature of his thinking—that is, its high degree of unity, consistency, and coherence across a wide variety of domains. In particular, I argue that in the Posterior Analytics Aristotle developed the Socratic ‘what is it?’ question into a theory of the connections among essence, necessity, and explanation, and that he systematically implemented this theory in his practice of explanation in the biological works, especially in the Parts of Animals. I also aim to show that this feature of Aristotle’s system continues to be of relevance to philosophy today.