In several places Aristotle distinguishes four types of cause, or explanation. First, he says, there is that of which and out of which a thing is made, such as the bronze of a statue. This is called the material cause. Second, there is the form or pattern of a thing, which may be expressed in its definition; Aristotle’s example is the proportion of the length of two strings in a lyre, which is the formal cause of one note’s being the octave of another. The third type of cause is the origin of a change or state of rest in something; this is often called the “efficient cause.” Aristotle gives as examples a person reaching a decision, a father begetting a child, a sculptor carving a statue, and a doctor healing a patient. The fourth and last type of cause is the end or goal of a thing—that for the sake of which a thing is done. This is known as the “final cause.”
Although Aristotle gives mathematical examples of formal causes, the forms whose causation interests him most are the substantial forms of living beings. In these cases substantial form is the structure or organization of the being as a whole, as well as of its various parts; it is this structure that explains the being’s life cycle and characteristic activities. In these cases, in fact, formal and final causes coincide, the mature realization of natural form being the end to which the activities of the organism tend. The growth and development of the various parts of a living being, such as the root of a tree or the heart of a sheep, can be understood only as the actualization of a certain structure for the purpose of performing a certain biological function.
For Aristotle, “being” is whatever is anything whatever. Whenever Aristotle explains the meaning of being, he does so by explaining the sense of the Greek verb to be. Being contains whatever items can be the subjects of true propositions containing the word is, whether or not the is is followed by a predicate. Thus, both Socrates is and Socrates is wise say something about being. Every being in any category other than substance is a property or a modification of substance. For this reason, Aristotle says that the study of substance is the way to understand the nature of being. The books of the Metaphysics in which he undertakes this investigation, VII through IX, are among the most difficult of his writings.
Aristotle gives two superficially conflicting accounts of the subject matter of first philosophy. According to one account, it is the discipline “which theorizes about being qua being, and the things which belong to being taken in itself”; unlike the special sciences, it deals with the most general features of beings, insofar as they are beings. On the other account, first philosophy deals with a particular kind of being, namely, divine, independent, and immutable substance; for this reason he sometimes calls the discipline “theology.”
It is important to note that these accounts are not simply two different descriptions of “being qua being.” There is, indeed, no such thing as being qua being; there are only different ways of studying being. When one studies human physiology, for example, one studies humans qua animals—that is to say, one studies the structures and functions that humans have in common with animals. But of course there is no such entity as a “human qua animal.” Similarly, to study something as a being is to study it in virtue of what it has in common with all other things. To study the universe as being is to study it as a single overarching system, embracing all the causes of things coming into being and remaining in existence.
The unmoved mover
The way in which Aristotle seeks to show that the universe is a single causal system is through an examination of the notion of movement, which finds its culmination in Book XI of the Metaphysics. As noted above, motion, for Aristotle, refers to change in any of several different categories. Aristotle’s fundamental principle is that everything that is in motion is moved by something else, and he offers a number of (unconvincing) arguments to this effect. He then argues that there cannot be an infinite series of moved movers. If it is true that when A is in motion there must be some B that moves A, then if B is itself in motion there must be some C moving B, and so on. This series cannot go on forever, and so it must come to a halt in some X that is a cause of motion but does not move itself—an unmoved mover.
Since the motion it causes is everlasting, this X must itself be an eternal substance. It must lack matter, for it cannot come into existence or go out of existence by turning into anything else. It must also lack potentiality, for the mere power to cause motion would not ensure the sempiternity of motion. It must, therefore, be pure actuality (energeia). Although the revolving heavens, for Aristotle, lack the possibility of substantial change, they possess potentiality, because each heavenly body has the power to move elsewhere in its diurnal round. Since these bodies are in motion, they need a mover, and this is a motionless mover. Such a mover could not act as an efficient cause, because that would involve a change in itself, but it can act as a final cause—an object of love—because being loved does not involve any change in the beloved. The stars and planets seek to imitate the perfection of the unmoved mover by moving about the Earth in a circle, the most perfect of shapes. For this to be the case, of course, the heavenly bodies must have souls capable of feeling love for the unmoved mover. “On such a principle,” Aristotle says, “depend the heavens and the world of nature.”
Aristotle is prepared to call the unmoved mover “God.” The life of God, he says, must be like the very best of human lives. The delight that a human being takes in the sublimest moments of philosophical contemplation is in God a perpetual state. What, Aristotle asks, does God think of? He must think of something—otherwise, he is no better than a sleeping human—and whatever he is thinking of, he must think of eternally. Either he thinks about himself, or he thinks about something else. But the value of a thought depends on the value of what it is a thought of, so, if God were thinking of anything other than himself, he would be somehow degraded. So he must be thinking of himself, the supreme being, and his life is a thinking of thinking (noesis noeseos).
This conclusion has been much debated. Some have regarded it as a sublime truth; others have thought it a piece of exquisite nonsense. Among those who have taken the latter view, some have considered it the supreme absurdity of Aristotle’s system, and others have held that Aristotle himself intended it as a reductio ad absurdum. Whatever the truth about the object of thought of the unmoved mover, it seems clear that it does not include the contingent affairs of individual human beings.
Thus, at the supreme point of Aristotle’s causal hierarchy stand the heavenly movers, moved and unmoved, which are the final cause of all generation and corruption. And this is why metaphysics can be called by two such different names. When Aristotle says that first philosophy studies the whole of being, he is describing it by indicating the field it is to explain; when he says that it is the science of the divine, he is describing it by indicating its ultimate principles of explanation. Thus, first philosophy is both the science of being qua being and also theology.