Aristotle's study of causes in Book II of the Physics is mostly accomplished with chapter 7. Chapters 8 and 9, which we have examined, do not speak of further kinds of causes, the one being a defense of final cause, which had already been put forward, the other a delineation of necessity in nature.
Chapter 7, on which we have not yet remarked, inquires about the principles of demonstration in the philosophy of nature, the causes (whether all or some) the natural philosopher should seek and analyze. "Since the causes are four, the business of the natural philosopher," says Aristotle, "is to know about them all; and in answer to the 'why' in the science of nature, he will reply with all four: the matter, the form, the mover, the end." 42 Thus, in physics (in Aristotle's meaning) there are four kinds of explanation, according to the four kinds of causes.
Aristotle, however, does not leave it at that. Having made his point, he seems to temper it in the next sentence. Form, mover, and end, he goes on to say, often coincide, "for the essence or 'what' and the end are one, and the proximate source of motion is identical in species with them (for man engenders man), and so, in general, are all moved movers." 43 This is a striking passage; it seems to indicate that Aristotle was disposed to narrow the methods of explanation to two. Form and end, when realized, are one, and the form by which the agent acts in the process of generation is like the form the agent seeks to introduce in matter. Thus, if form and end are one, and the agent is sometimes of the same species as the form introduced in matter, all three may be considered as one, or under one aspect - two things equal to a third equal each other.44 This analysis would leave us with two, and not four, truly distinctive types of explanation in physical philosophy, one taking its evidence from the material elements of a thing (material cause), the other from its formal structure (form and its properties), which, in the ultimate reckoning, receives its determination from final cause.45 The early Physicists had centered their attention on material cause; the object of their search had been the primordial substance or its basic elements. Aristotle is more with Plato, seeking in form and end the greater revelation. Foremost, however, is the end, whether in explanation or being, in thought or reality.
Yet it should not be assumed that the reduction of method to two types of explanation is absolute. Aristotle does not retract the assertion that the philosopher of nature should produce proofs from all the causes, and that each type of demonstration has its own character. The proof from efficient cause is frequently used, and this cannot be reduced to material cause, to the allocation or disposition of the elements. Nor, though for different reasons, can it be assimilated to the exemplary causality of form. But the efficient cause depends ultimately on the prime mover, which, be it noted, moves secondary movers by the "desire" it evokes, hence by final causality. Thus, final cause always stands first, and the light it sheds surpasses every other.
This, then, is Aristotle's general theory of demonstration or explanation in natural philosophy. One last question suggests itself. What would a modern scientist make of it, or how do Aristotle's theory and method compare with modern conceptions? Final causes, no one doubts, have lost much of their former standing in the study of nature. A possible exception is biology, where, often by different name, they still seem to be recognized. But recognized or not, they are and remain the foremost causes of things. If, as intimated, they have fallen from scientific grace, the reason may well be that they are more difficult to discover than other causes, perhaps much more difficult than the ancients realized.
In addition one can maintain the validity, or even the superiority, of demonstration by final cause without as- signing it priority in practice. Often one's purpose is met, or one's progress halted, before coming to final cause. Thus, it may at times be necessary to settle for more immediate demonstrations, whether by efficient cause (seeking the antecedents to a given effect), by material cause (theorizing the structure of the elements), or by formal cause (as in mathematical analysis). This granted, there are no basic incompatibilities between Aristotle and modern scientists. His ideas on what to look for in nature and what kinds of proof to produce will readily harmonize with their practice and procedures.
We end this chapter with a diagram. It depicts the chain of causality in nature as envisaged by Aristotle. Heading the chain is final causality, which results in the hypothetical determinism of the other causes. Each of these produces a specific effect by its specific causality. Completing the chain is the chance effect of accidental causality. Thus, the diagram portrays the sum and substance of Aristotle's doctrine of causality:
Final causality (hypothetical) → determinism of antecedent causes → proper effect by proper causality & chance effect by accidental causality