 This entry on Aristotle’s biology and its philosophy closes with a puzzle about the organization of his biological works when compared with the remainder of the Aristotelian corpus. I began by noting that it is a fundamental aspect of Aristotle’s theory of science that investigation proceeds from a stage where one establishes the existence of the things being studied and the facts about them to a stage where the focus is on causal explanations by reference to the natures and essences of those things. It seems clear that the biological works honor this distinction, and do so self-consciously. Yet Aristotle never suggests, in the Posterior Analytics or anywhere else, that these stages should be represented by distinct treatises devoted to the same subject. Indeed, in no other domain does Aristotle do this: whether we look to meteorology, cosmology, psychology, ethics, drama or rhetoric, we find single treatises presenting both the facts and their explanation. Nor can this puzzle be easily dismissed as an artifact of later editing—as I have pointed out a number of times, these treatises have a most interesting and consistent pattern of cross-referencing. The treatises that report the results of causal investigations regularly refer to HA (and the dissections) for more detail regarding the facts being explained; the HA, in contrast, never refers to the causal treatises. Moreover, the striking avoidance in the Historia Animalium of all of the concepts associated with definition and explanation cannot reasonably be laid at the feet of some imagined later editor. We are left, then, to ponder what it was about the investigation of animals that led Aristotle to take a methodological distinction regarding stages of investigation and reify it in methodologically distinct treatises devoted to these different stages.