 No one tries to live well for the sake of some further goal; rather, being eudaimon is the highest end, and all subordinate goals—health, wealth, and other such resources—are sought because they promote well-being, not because they are what well-being consists in. But unless we can determine which good or goods happiness consists in, it is of little use to acknowledge that it is the highest end. To resolve this issue, Aristotle asks what the ergon (“function”, “task”, “work”) of a human being is, and argues that it consists in activity of the rational part of the soul in accordance with virtue (1097b22–1098a20). One important component of this argument is expressed in terms of distinctions he makes in his psychological and biological works. The soul is analyzed into a connected series of capacities: the nutritive soul is responsible for growth and reproduction, the locomotive soul for motion, the perceptive soul for perception, and so on. The biological fact Aristotle makes use of is that human beings are the only species that has not only these lower capacities but a rational soul as well. The good of a human being must have something to do with being human; and what sets humanity off from other species, giving us the potential to live a better life, is our capacity to guide ourselves by using reason. If we use reason well, we live well as human beings; or, to be more precise, using reason well over the course of a full life is what happiness consists in. Doing anything well requires virtue or excellence, and therefore living well consists in activities caused by the rational soul in accordance with virtue or excellence.