 In sections 4, 5 and 6, we will explore the ways in which Aristotle systematically organizes and explains an extraordinary body of information about animal anatomy, physiology and development. But Aristotle was able to accomplish what he did in zoology because he had given a great deal of thought to the nature of scientific inquiry. How does one progress from the superficial and unorganized state of everyday experience toward organized scientific knowledge? To answer this question, you need a concept of the goal to be achieved, and Aristotle developed such a concept in his Prior and Posterior Analytics (henceforth abbreviated as APr. and APo., respectively). The goal of scientific inquiry, he argued, was a system of concepts and propositions organized hierarchically, ultimately resting on knowledge of the essential natures of the objects of study and certain other necessary first principles. These definitions and principles form the basis of causal explanations of all the other universal truths within the domain of study. Those other universal truths should identify attributes belonging to a subject per se, in virtue of that subject’s nature. The example he uses when he introduces his account of scientific demonstration to illustrate such propositions is from geometry: having interior angles equal to two right angles belongs to all and only triangles in virtue of their being triangles (APo. I 4, 5). This attribute belongs to all equilateral triangles as well—not, however, because they are equilateral, but because they are triangles. Thus scientific knowledge of such a proposition, knowledge that displays the reason why any triangle has this property, must explain why this property belongs to triangles as such. The explanation, of course, will appeal to the essential character of three-sided rectilinear plane figures, i.e. to what it is to be a triangle.