In the last two chapters of Book II 83 Aristotle again comes to grips with the mechanist theories of his predecessors. This time it is their philosophy of cause that is called to account. The mechanists, by and large, believed that every cause-and-effect sequence resolves itself into a chain of necessary, blind determinations. "Since the hot is by nature such, and the cold by nature such, and similarly other things, therefore this kind of thing and this kind of change necessarily follow from them." 34 This, in substance, is their argument, which, if true, amounts to the suppression of final cause. "Why," to sample their case further, "should not nature act, not in view of an end or because it is better, but just as Zeus causes the rain, not to make the corn grow, but of necessity? For, what evaporates into air must cool, and when cooled must become water and fall again. That the corn in consequence grows, is accidental. So also when, on the other hand, the corn is spoiled on the threshing floor, it was not for this that the rain fell - it was just incidental [to the rain]." 35
Aristotle's answer to this position begins with a thoroughgoing defense of finality in nature. That done, he goes on to explain how finality nevertheless accords with a degree of necessity in causal sequences. Eliminated, however, is the absolute determinism which the mechanists attribute to the cause-and-effect relation in nature.
1. Finality in Nature
Aristotle's demonstration of finality in nature finds him at his resourceful best. Three arguments stand out. The first is drawn from the fact of chance. Some things are due to chance, they happen but rarely. On the other hand, what happens as a rule, or regularly, cannot be the result of chance, hence must occur in view of an end. If, in other words, there is chance, there is finality. The coexistence in nature of the seldom and the constant, of the regular and the irregular, is unexplainable unless there is both finality and chance.
Another argument is seen in art and nature following similar courses: art imitates nature. A doctor in his treatment follows nature's lead. If, then, as is manifestly true, finality is in art, it must also be in nature. Thirdly, Aristotle finds finality in the way that animals and even plants adapt themselves to their needs and functions. Neither of these act by intelligence. The swallow building its nest, the spider spinning its web, the plant pushing its roots down into the nourishing soil, all these act by nature yet with obvious finality, or purpose of action.
These, in sum, are Aristotle's arguments." To develop them in detail, or to compare and contrast them with modem modes of scientific thought would take us too far afield. But pry and prick as we will, we should not find them gravely vulnerable. The core of these arguments, if not the trim, endures. As a matter of fact, the existence of final cause can be established even more directly, by the metaphysical approach. The starting point in that case is efficient cause, and the argument turns on the prerequisites of this cause. As mentioned earlier, the prime precondition of efficient cause is final cause, for no agent acts without an end. St. Thomas presents this thought as follows: "An agent dots not move except with intention of an end. For, if an agent were not set on some definite effect, it would not do one thing instead of another. Hence, in order that it produce a determinate effect it must be fixed on some definite thing, which has the nature of an end." 37
In fine, every single instance of efficient causality, which is to say every act of any kind, necessarily implies finality, or final cause.
It may be objected that nature cannot act for an end because it lacks intelligence, hence cannot deliberate and decide. To this the answer is that there are two ways of moving toward an end. One, as St. Thomas explains, is the way of rational creatures, who know their end and move themselves toward it. The other is the way of irrational creatures, who are borne toward their end by the transcendent motion of a higher intelligence. The former act (agunt) in view of an end; the latter are moved (aguntur) toward their end. For, observes St. Thomas, "the entire irrational world is related to God as an instrument is to a principal agent." 38
To sum up, nature is unquestionably endowed with finality. But this is not to say that one can always identify the specific end of each thing and each activity in nature.39
2. Necessity in Nature
The case for finality is now made. But does this mean that all necessity is eliminated from nature? And if not, what manner of necessity does nature admit? Necessity, as Aristotle explains it, is twofold: absolute and hypothetical. Absolute necessity is one that depends on pre-existent causes: "quae dependet ex causis prioribus," to use St. Thomas' wording. St. Thomas, moreover, notes that this necessity is found in three kinds of causality." It is found in material causality, as may be seen in an animal, which is necessarily corruptible because it is composed of contraries. Again, absolute necessity occurs in formal causality. Thus, what flows from or reduces to the definition of a thing is absolutely necessary; a man, for example, is necessarily rational (this pertains to his definition), and the interior angles of a triangle equal two right angles (this flows from the definition of a triangle). Lastly, this kind of necessity exists in the realm of efficient causality, since the action of the agent entails the effect. So, in St. Thomas' example, the alternation of day with night is necessary on account of the sun's movement.
Hypothetical necessity, on the other hand, is tied to a condition, to something not yet effected: "necessitatem ab eo quod est posterius in esse" is St. Thomas' phrasing. It is the necessity exemplified in the statement: "This thing is necessary if that thing is to be made or done."
Aristotle takes exception with those who acknowledge only absolute necessity in nature. Not only, he declares, is there the necessity of finality, which is hypothetical or conditional, but this necessity is preponderant. The predominant reason why a house, to take his example, comes into being is not because certain materials are put together; rather, the materials are assembled and put together because a house was decided on. The end (final cause) came first. Similarly, we should more correctly say, not that a saw cuts because it has iron teeth, but it was given iron teeth so it would cut. In all cases of this kind the prime source of necessity lies in final cause, which may or may not be put. Hence the necessity is hypothetical, conditional.
Ultimately, therefore, all necessity in nature rests on final cause. But, as mentioned a moment ago, necessity does reach into the other causes, too. To produce a certain kind of thing it will be necessary to use a certain kind of materials; or, this kind of agent must be had to perform this kind of work. The attainment of the end does, in some manner and measure, depend on matter and the other preexistent causes. This simply means - and we shall come back to the point in the next heading - that nature is a complex reality, that the whole explanation of its course and events must be sought in all the causes. The end, however, is the principal cause and condition. All others are subordinate and secondary. Whatever they contribute to a thing, they contribute by virtue of final cause; and, inversely, whatever a thing owes to them, it owes still more to final cause. This, in effect, is the burden of the following excerpt from St. Thomas' Commentary:
Therefore, it is clear that that is said to be necessary in natural things which is like matter or the material motion, and the reason for this necessity is from the end, because by reason of the end it is necessary that there be such matter. Also, the philosopher of nature should give both causes, namely the material and the final, but more the final, because the end is the cause of the matter but the opposite is not true. It is not true that the end is such because the matter is such, rather the matter is such because the end is such 41
For Aristotle, then, there is a kind of determinism in nature, but its inmost reason lies in finality, and hence in intelligence. It is a determinism that leaves room for accidental causality, and so for things of chance. All told, Aristotle's theory of finality and necessity makes for an explanatory apparatus that is remarkably flexible, encompassing nature in its several aspects.