Aristotle’s argument in basic terms is as follows: a watch has a function and its goodness resides in that function; if man has a function then his goodness lies in performing that function well. For a watch to perform its function it uses the mechanisms within itself to achieve this; each of man’s bodily organs have a function and so therefore man must have a function and this function is man’s distinguishing feature: rationality. The chief good for man is a life following or implying a rational principle and to use that reason together with certain virtues.
A problem with Aristotle’s belief is his claim that each of a man’s bodily organs have a function and so therefore man must have a function. Not everything in the world has a clear function or a determined end. For example, a rose does not have a clear function other than the functions we think of for it (its beauty and its fragrance) but this does not add anything factual to it. In comparison, the function of our eyes give us the ability to perceive the world but this adds nothing factually to simply saying that our eyes causes us to perceive. When we speak about function we give it a normative status to causation but this is subjective to every individual. This works for all teleological ideas and it reflects our own interests.
As for teleological arguments, they can only be defended, mainly, by religion and also by anthropomorphic ideas of nature. For example, Thomas Aquinas believed that natural law was not made up by humans but rather an unchanging rule or pattern which is there for human beings to discover. Aquinas says that natural law is so complex that it had to have been designed by a higher power and he stated that the only plausible answer is God. However, using God as the answer to the existence and aim of human beings is a weak argument. Jean-Paul Sartre believes in the concept that “existence precedes essence” and that the idea that existence precedes essence means that a human being, as well as human reality, exists prior to any concepts of values or morals. A person is born a blank slate and humanity has no universal, fixed values or ethics common to all of mankind. Since no essence or definition exists of what is means to "be human," a person must form their own conception of existence by taking control of responsibility for their actions and choices. Therefore, a human being gains their essence through their own choices and actions. It is solely through the process of living that a person defines themselves. He uses the example of a paper knife saying that “one cannot suppose that a man would produce a paper knife without knowing what it is for”. A paper knife has essence before existence because it is designed for a specific purpose. Human beings do not; they have existence before essence because they are not designed with a specific purpose. Consequently, this challenges the function argument in that human beings do not have a certain end or a definite function; our function in life is made up as we go through life.
Another problem with the function argument is presented by the is/ought fallacy. David Hume argued that there is a philosophical problem in believing that because something is the case it ought to be the case; he calls this the is/ought fallacy. For example, slavery exists but the fact that it is a reality does not mean that it ought to be. Abortion is a reality and some would argue that it is right to abort in certain circumstances but that does not mean we ought to. In this case, it may be a fact that humans possess reason but it does not logically follow that we ought to exercise our reason to live a fulfilled life. Professor Richard Norman says: “why then from the fact that rational activity is distinctively human should it follow that we ought to live according to reason?”
An additional problem is, there are many distinctively human things that animals cannot do, why is reason the only characteristic Aristotle focuses upon? For example, we can gamble, give to charity, make art and become intoxicated by drugs but that does not mean that any of these are our functions. On what grounds does Aristotle use that animals cannot use reason? Surely what we call “reason” is no more than instinctive response but on a conscious level than any action in the animal kingdom. Aristotle could simply argue that these are all examples of humans not using their reason well because a tyrant, terrorist or gambler is using their reason but not in conjunction with their virtues. A good example of this would be the terrorist Osama bin Laden who thought he was doing good for the world where in reality he was creating devastation.
In conclusion, although Aristotle’s belief of man’s function in life gives us as human beings something to aim for (eudaimonia) it does not mean that man definitely has a function in life. Just because our organs work in a certain way does not mean our body must work towards something, and if our bodies are indeed working towards something then why must it be towards eudaimonia? As Sartre says, our function could be made up as we go through life. Why must we live life according to reason? Also, why must it just be reason we function upon? It is these questions that pose a problem to Aristotle’s function argument and therefore make his claim flawed.