In order to advance your ability to understand and apply ethics, you must provide a plausible answer to this question: “What makes an action good?” If you cannot state what makes an action right or wrong in general, you cannot participate in a meaningful discussion of any moral issue. If you cannot say why an action is right or wrong, then (for example), a debate about abortion is simply an exercise to determine who can talk louder.
However, if you can identify what it is that makes an action right or wrong, you can use that standard to evaluate any potential action. Aristotle’s Function Argument provides an answer to "What makes an action good?" (i.e., what makes an act right or wrong), and to "Where does value comes from?"
The Function Argument takes this form: Start with something you know can be good, or not. For example, a steak knife. Next, ask, “When is it good?” The steak knife is not good when it cuts meat, but when it cuts meat well. Something is good when it excels at its function. This is true of everything.
When considering examples (e.g., a good meal, a good cell phone), it quickly becomes clear that a thing’s function is tied to what kind of thing it is. Function follows essence. What makes a thing what it is tells us how that thing should be.
Since no one wants to be an excellent steak knife or an excellent puppy, we must consider the human essence, which is to be a rational animal. To be a good human is to excel at being a rational animal or, more precisely, to cultivate the virtues conducive to excellence as a rational animal.
Aristotle’s Function Argument connects function to excellence. The word Aristotle used is “arête,” which is translated as "virtue" or "personal excellence." Aristotelian ethics is concerned with answering this question — “What kind of person should I be?” — rather than “How should I act?” (the question that is now the norm in ethics). These two questions are clearly related, but differ in their primary consideration. For Aristotle, character was primary. Actions are evaluated based on how they inform your character (e.g., Does an action make you greedy? Smug? Generous? Brave?).
Aristotle used biology to answer “What kind of person should I be?” in a way that, when we must act, we will act according to our individual characters. The kind of character to be evaluated is based on what kind of thing we are (i.e., rational animals). By “rational,” Aristotle meant something very specific. Since rational is of our essence, it is what separates humans from other animals. Humans are the kind of thing that can think abstractly (not just of what to do in a particular case, but in general). Humans can plan the best ways to achieve their goals (which are focused on their happiness).
For this reason, an essential component of rational choice is deliberation. We deliberate on how we can best achieve the good life, and on what kind of person each of us should be. This tells us the way we should be, and the virtues we should have.
Once we have determined the virtues we must cultivate, and have cultivated them, we must begin to manifest them. This is not easy, because virtue comes in degrees. For example, courage is a virtue we should cultivate. Rational deliberation indicates that we should be courageous people, as part of human excellence. However, we must determine how courageous we should be.
To answer this question, Aristotle provided the Doctrine of the Mean. Like other Aristotelian arguments, it begins with something we know: First, take something we know can be a virtue, such as courage. Next, ask "when is courage not a virtue (i.e., when is it a vice)?" Too little courage and you are a coward. Too much courage and you are foolhardy. (Note that there is nothing admirable about picking fights with tough people, or taking other unnecessary risks.) Courage, therefore, must be cultivated as a mean between extremes.
As with the Function Argument, Aristotle used a specific example for a general claim: Any virtue is turned into a vice when developed to the point of excess or deficiency.
Aristotle indicated that this ideal is a rational mean, not a numeric mean. What does this mean? Let's consider an example. It is a virtue to be financially productive (i.e., the trait involved in becoming rich)? What’s the least I can earn in a year? Zero. What is the most? Currently, the record is about $12.7 billion. The numeric mean of financial productivity would be a trait that leads to earning $6.35 billion dollars a year, but it would be absurd to make this a personal goal. It is clearly excessive. What did Aristotle mean by "rational mean?" To answer, recall rational deliberation, and the process involved in determining what kind of person I should be. If I dedicate too much of my effort to developing my financial productivity, I will fall short of the good life because other aspects of my life will suffer. I will not expend enough effort to develop other positive traits.