This is a study of substantial being and the role it plays in Aristotle’s biology. Substantial being in the biological context can, as an analytic first pass, be defined as the essence or what it is to be an animal of a certain kind. According to the framework I develop, substantial being plays two major roles. The first of these is explanatory, particularly within the method of division. This method, sketched out by Plato in the so-called “late” dialogues, is discussed in detail in two works, the Parts of Animals and Posterior Analytics. The idea in Plato is that by taking a general kind and making two mutually incompatible divisions, we can reach the essence of something. Aristotle wrestled with this Academic legacy, and what we find in h...