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Aristotle wrote Nicomachean Ethics in order to discover the nature of the ultimate good in human life. Common opinion states that this is simply a good life (or more precisely eudaimonia), and so he takes it as given that this is true. The question, then, is what that means exactly, and how we should strive for it, because on this there is no common opinion. Aristotle dismisses many views, but believes that there is merit in the opinions of the refined, which state that the best life is one of honor or virtue, and in the belief that a life of study is a good life. In support of these, and to refine them, Aristotle offers the function argument.
Beginning from the premise that everything is in the world for a reason, we can conclude that people are in the world for a specific function as well. A good person is one who fulfills this role, just as a good knife is one that cuts well, a good gun is one that fires with accuracy, a good bee one that makes sweet honey. Given that people have a function, Aristotle then asks what this function is. What can human beings do better than anything else in the world?
By process of elimination we arrive at an answer. We do not fulfill our purpose simply by being alive, for plants are alive as well. He next brings up the idea that the human function is a life of sense-perception, or awareness of surroundings. But all animals, from the spider to the horse, also have lives in which they see and feel things around them, so it cannot be this either. The only option left, then, is that we exist to fulfill our reason in our actions. For nothing else, so far as we know, is capable of demonstrating reason.
This answer, though, requires still more clarification. For what, exactly, would constitute demonstrating reason? And it is here that the function argument finally supports the opinions of the wise. First, Aristotle divides the role of reasoning into two parts. There is the part of reason that controls emotion, which denotes how to act correctly in each given situation. Perfection of such reason denotes several "virtues of character." Reason also exerts itself in our understanding of the world around us, and the virtues associated with this are termed "virtues of thought."
In every given situation, Aristotle believes there is a golden mean of action between two extremes, one of excess and one of deficiency. Reason is applied to discover where this mean lies. Thus, to use the virtue of courage as an example, there are some situations in which courage would include standing and fighting, without fear, as would be the case in a battle. There are others, however, such as the time when a man would be outnumbered one-hundred to one, in which fighting would be suicide, and in such a case the appropriate response might be flight. The vice of foolhardiness lies on one side. Fighting without fear in every circumstance is foolish, a demonstration of an excess of bravery. On the other side is the vice of cowardice. Running in every situation is as deplorable as its opposite. Thus, a true reasoning man would discern the most appropriate level of courage in every situation and, after thought, act as his reason commanded.
Courage, however, is one of many virtues of character, and cannot alone satisfy the requirements for leading a good life. For, one who is courageous but also malicious is certainly not virtuous, as he uses his courage in the pursuit of evil. A truly virtuous person would demonstrate their reasoning in every characteristic.
It is important to note that a truly virtuous man, once he had decided how to act, would do so with a free heart. One who acted unhappily would not then be demonstrating reason, for to truly understand why such a response is the best is to want to respond that way. With this, Aristotle supports the view that a good life is one of virtue and honor.
The second set of virtues is virtues of thought, and it is with these that we are able to lead the best kind of life. The most important example of a virtue of thought is theoretical wisdom. Wisdom is achieved when a person has two things, understanding of the world around them and an ability to apply this understanding to new situations. Thus, a scientist who makes a hypothesis based on current observations which then predicts the results of new experiments demonstrates wisdom. This supports the common belief that a life of study is a good life.
In closing, Aristotle is interested in the function of human beings because he believes that the best kind of life is one in which we fulfill this function. This function is reason, and is broken into two parts, virtues of character (i.e. courage) and virtues of thought (i.e. the pursuit of wisdom). The best kind of life then, involves both of these. A human being who demonstrates reason in choosing the golden mean in each action, and who understands the world around them and can make observations based on this understanding, is truly living, according to Aristotle, the best kind of life.