In Book II of Physics, a treatise on natural philosophy, Aristotle presents a teleological conception of nature, a view holding that purposive ends exist in nature and account for natural substances and their processes. Substances, according to Aristotle, are natural insofar as they possess a nature, an internal "principle and cause of motion and stability," (192B22) belonging primarily to that which is said to posses the nature and to be made natural by it (192B23). Aristotle outlines four explanations for being, which include material, formal, efficient and teleological justifications for things, and shows that nature comprises all four of these explanations. The teleological account of being, or the "for the sake of" cause, is controversial when said of nature because it challenges many pre-Aristotelian accounts of being, most notably that of Empedocles, who believes there are no goals in nature, but rather coincidental ecological successes determined by chance permutations of elements. Aristotle wants to show that there are higher organizational principles or "forms," which he identifies as "natures" in natural things. He shows that nature is a "for the sake of" cause by arguing that natural things are not caused coincidentally but are for something, and that being for something, natural things and their sequences are directed towards their ends; towards what they are for; towards their form; and towards their nature.
What Makes Something Natural
Before explaining why nature is for something, Aristotle explains what it is that makes something natural, and what a final or "for the sake of" cause is. To be a natural thing is to have a nature, "a type of principle and cause of motion and stability within those things to which it primarily belongs in their own right and not coincidentally" (192B21). For example, a rose is natural thing because its primary source of change qua rose is internal. It becomes a rose from a smaller rose, and a smaller rose from a seed, by an internal source of change. Also, a mature rose drops its dead petals to preserve itself as a rose. On the other hand, a wall is not natural because it does not possess a nature qua wall; its primary source of change insofar as it is a wall is external – a mason. Aristotle also explains the notion of final or "for the sake of cause" as something's end or what something is for (194B33). To demonstrate, health is the final cause of going to the gymnasium; one goes to the gymnasium for the sake of health, and the facilities of the gymnasium are such that they enable one to get health.
Coincidental Causes and Empedocles' View
Aristotle considers another explanation of cause in terms of coincidence, identifying a coincidental cause as anything that coincides with a cause. Considering a housebuilder is the cause of a house (196B26), one of many coincidental causes of the house is a father, if the father coincides with the housebuilder. A coincidental final cause is the end of what is caused, but results only rarely and by coincidental effect; if Aristotle went to the gymnasium for his health but unexpectedly met a long lost friend from The Academy, health is the unqualified final cause of going to the gymnasium, but meeting the friend is a coincidental final cause (196B32).
Empedocles advocates coincidental causes, such as luck and chance, in nature, and supposes natural things and their parts, insofar as they are organisms and organs, possess only coincidental final causes. Empedocles would say that, in a dog, "the front teeth grow sharp for biting, and the back ones broad and useful for chewing" (198B24) because the dog and its parts were randomly composed of the elements Earth, Air, Fire and Water, and it is only coincidental to the simple properties of these elements that faculties for biting and chewing resulted.
Natural Things Are For Something
Using these ideas of causation, Aristotle begins his argument against Empedocles' conception of coincidental causation in nature, showing that natural things have unqualified, "for the sake of" causes. Aristotle and Empedocles agree that organisms and their parts are natural things (I 198B35); however, Aristotle diverges from Empedocles' conception of causation in nature, proposing that natural things result either coincidentally, as Empedocles believes, or for something (II 199A4). Aristotle observes that natural things occur either always or usually (III 198B36), and that which is caused coincidentally occurs only rarely (IV 198B36). Making an inference from III and IV, it is shown that natural things are not caused coincidentally (V 199A5), and it follows from II and V that natural things are for something (VI 199A8); whereas Empedocles believes "[Earth, Air, Water and Fire] began to fall together, however they chanced to meet," (31B59) creating natural things virtually at random, whose every "useful result was coincidental" (198B26), Aristotle argues soundly that natural things are for something.
Natural Sequences Are For Their Ends
Aristotle argues further that natural sequences are for their ends. First, it is stated that whenever a sequence of deeds has an end, the sequence was for the end (I 199A9). Aristotle then considers a sequence of deeds that would lead to a natural end, and, equivalently, a natural sequence that would lead to an artificial end: for example, a house coming to be naturally. According to Aristotle, the house would come to be by nature in the same way it would by craft because the end is the same (199A12), and he forms the premise that the same end would come to be from either type of sequence, artificial or natural, in the same way if nothing were to prevent it (II 199A16). Aristotle observes that art either imitates or completes nature (III 199B17), and that art is for something (IV 199A17), and infers from III and IV that natural sequences are for something (V 199B12). Further, it follows from I and V that as art and other deeds are for their ends, so are natural sequences for their ends (VI). After showing what it is to be a natural thing, and that natural things and their processes are for their ends, Aristotle must show what the ends of natural things and their processes are to complete his argument against Empedocles' for nature as a "for the sake of" cause.
Nature is Form
Aristotle concludes his argument for a teleological account of nature by showing that nature is more form than matter, that form is an end or goal of natural things and their processes, and, ultimately, that nature is a "for the sake of" cause. Earlier in Physics, Aristotle shows that nature can be considered both "the primary matter that is a subject for each thing" (193A29) and "the shape and form of things," (193B4) and he concludes that nature is more form than matter (193B8). He arrives at this conclusion by arguing that a man, as an example of a natural thing coming to be by its nature or internal principle of change, comes to be from what is also a man, not what is potentially a man (193B12), referring to Empedocles' elements. Similarly, Aristotle asks, "what is it then, that grows? Not what it is growing from, but what it is growing into. Therefore, the [form] is the nature" (193B16).
Aristotle culminates his argument for nature as a "for the sake of" cause by restating nature as a duality of form and matter (I 199A31). Drawing from examples of natural sequences regularly occurring for the benefit of the form (what is grown into) (193B16), Aristotle says that form is the end of natural things and their processes (II 199A32); a puppy develops teeth which allow it to better nourish itself and to grow into a dog. Aristotle recalls his previous conclusions that natural things and their processes are for their ends (III 199A32), and infers from II and III that they are for their forms (IV), and it follows from I and IV that they are for their natures (V). If nature is what natural things are for, and the processes of natural things are such that they result in nature, nature is a "for the sake of" cause (VI). This argument successfully builds upon Aristotle's previous arguments for nature as form, for natural things as for something, and for natural things and their processes as for their ends, adding that form is "what is grown into" (193B16) and is therefore the end of that which grows.
Conclusion
Aristotle offers a wonderfully constructed argument against Empedocles for nature as a "for the sake of" cause, but objections are not difficult to find. Empedocles would argue that anomalies in nature, such as "neckless faces" (31B57) or "man-headed calves" (198B34), show that nature is coincidental to chance permutations of matter. Aristotle answers this objection by saying that nature is what is usual, and that there is room for variance of principles; nevertheless, nature is a "for the sake of" cause.
