The Aristotelian Ethics all aim to begin with approximate but uncontroversial starting points. In the Nicomachean Ethics Aristotle says explicitly that one must begin with what is familiar to us, and "the that" or "the fact that" (NE I.1095b2-13). Ancient commentators agree that what Aristotle means here is that his treatise must rely upon practical, everyday knowledge of virtuous actions as the starting points of his inquiry, and that he is supposing that his readers have some kind of experience-based understanding of such actions, and that they value noble and just actions to at least some degree.
Elsewhere, Aristotle also seems to rely upon common conceptions of how the world works. In fact, some regard his ethical inquiries as using a method that relies upon popular opinion (his so-called "endoxic method" from the Grk. endoxa). There is some dispute, however, about exactly how such common conceptions fit into Aristotle's method in his ethical treatises, particularly since he also makes use of more formal arguments, especially the so-called "function argument," which is described below.
Aristotle describes popular accounts about what kind of life would be a happy one by classifying them into three most common types: a life dedicated to vulgar pleasure; a life dedicated to fame and honor; and a life dedicated to contemplation (NE I.1095b17-19). To reach his own conclusion about the best life, however, Aristotle tries to isolate the function of humans. The argument he develops here is accordingly widely known as "the function argument," and is among the most-discussed arguments made by any ancient philosopher. He argues that while humans undergo nutrition and growth, so do other living things, and while humans are capable of perception, this is shared with animals (NE I.1098b22-1098a15). Thus neither of these characteristics is particular to humans. According to Aristotle, what remains and what is distinctively human is reason. Thus he concludes that the human function is some kind of excellent exercise of the intellect. And, since Aristotle thinks that practical wisdom rules over the character excellences, exercising such excellences is one way to exercise reason and thus fulfill the human function.
One common objection to Aristotle's function argument is that it uses descriptive or factual premises to derive conclusions about what is good. Such arguments are often thought to run afoul of the is-ought gap.