Aristotle's thought is consistently teleological: everything is always changing and moving, and has some aim, goal or purpose. To borrow from Newtonian physics, we might say that everything has potential which may be actualized. An acorn is potentially and oak tree for example. The process of change and motion which the acorn undertakes is directed at realizing this potential. Aristotle believed that things in nature occur because they serve a purpose. He maintains that organisms develop as they do because they have a natural goal or telos in Greek. "Nature", writes Aristotle, is "a 'principle of motion and change' "(Physics, 200b1), where "motion" or "movement" (or change as we discussed in our classroom) describes the "fulfillment of what exists potentially, in so far as it exists potentially"(201a) in a thing.
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But is there any reason for saying nature has a goal? Why cannot the rain rain and the sun shine, not because the sky is cloudy or clear but just by chance? Empedocles argued for a theory of natural selection on the basis of chance. The survival of the fittest means that those who happen to be more fit survive longer. The less fit perish. Aristotle rejects any theory of evolution. Things either occur by chance or they occur "always or for the most part, "which is the opposite of chance. You must admit that things that occur "always or for the most part" occur either by chance, or not by chance. If they occur not by chance, then they occur for a purpose. Let's take the example of monsters. Monsters occur by chance because they are not among those things that are always or for the most part. Man, on the other hand, survives because he is meant to survive. To argue that he is a result of chance is to argue that he does not exist always or for the most part, but only sometimes. This, of course, is absurd. Because most things in nature seem to occur most of the time and exhibit a pattern of change which can be broken up into the four causes, Aristotle argues that nature must have a purpose. Order and conformity to type infer purpose.
Aristotle goes on in Book II to make his explanation of purpose in nature more clear by relating natural purpose to artistic creation. In any process of human creation, there is a definite end to be achieved. In order to achieve that end, the artist must complete a series of steps to bring this end about. For example, if you want to build a house, there are certain steps you have to go through in order to bring the house into being. If those steps are not followed, you may very well end up with something that resembles a house but is not a house. Natural processes imitate nature in the way art works come into being. If the art if for an end, nature must even more be for an end. One has only to look at the work of swallows, ants or bees, who have no conscious purpose, to realize that they are nevertheless acting according to a purpose. Mutations are simply nature's failures, the miscarriage of purpose. If all had gone well, the monster would have been a man and that which resembles a house but is not a house would have been a house. When nature fulfills her purpose, man begets man and nothing else. The natural end of anything is to conform to its type, to become what it is designed to be.
In his zoological research Aristotle set forth his teleological view of nature based upon his observations. To explain a phenomenon, Aristotle said that one must discover its goal, to understand "that for the sake of which" the phenomenon in question existed. A simple example of this kink of explanations is the duck's webbed feet. According to Aristotle's reasoning, ducks have webbed feet for the sake of swimming which is an activity that supports the goal of a duck's existence. That goal is to find food in water so as to stay alive
