The general theory of science outlined in Aristotle’s Posterior Analytics mandates that the scientific enterprise proceeds in stages, and that the two main stages of any scientific inquiry are the collection of the relevant data followed by their explanation – the pre-explanatory and the explanatory stages of inquiry, respectively. Aristotle’s study of animals illustrates this methodological insight in an especially clear way. Moreover, the following epistemic principle controls Aristotle’s study of animals: the study of animals must start from the most organized and most determinate form of life and must take that as its starting point to generate results that can be subsequently extended to what is comparatively less organized and less articulate. This means that the study of animals must begin with a discussion of the human body. This methodological insight is at work in Aristotle’s History of Animals. However, its significance goes well beyond the stage of the collection and presentation of the relevant data; this chapter shows that this rule of inquiry also shapes the explanation of the zoological data in Parts of Animals, Progression of Animals, and Generation of Animals. The chapter also discusses the distortions created by the application of this rule of inquiry with a concentration on Aristotle’s explanation of animal locomotion.