When we correctly answer questions such as ‘Why does this event happen?’ or ‘Why is this object as it is?’, we state the cause (or explanation; aition) of the event or object. Aristotle believes that causes are multivocal (see Physics II 3; Metaphysics I 3). Different accounts of a cause correspond to different answers to why-questions about (for example) a statue. (1) ‘It is made of bronze’ states the material cause. (2) ‘It is a statue representing Pericles’ states the formal cause, by stating the definition that says what the thing is. (3) ‘A sculptor made it’ states the ‘source of change’, by mentioning the source of the process that brought the statue into being; later writers call this the ‘moving cause’ or ‘efficient cause’. (4) ‘It is made to represent Pericles’ states ‘that for the sake of which’, since it mentions the goal or end for the sake of which the statue was made; this is often called the ‘final’ (Latin finis; ‘end’) cause.
Each of the four causes answers a why-question. Sometimes (as in our example) a complete answer requires all four causes. Not all four, however, are always appropriate; the (universal) triangle, for example, has a formal cause, stating its definition, but no efficient cause, since it does not come into being, and no final cause, since it is not made to promote any goal or end.
Some have claimed that Aristotle’s ‘four causes’ are not really causes at all, pointing out that he takes an aition to be available even in cases where the why-question (for example, ‘Why do the interior angles of this figure add up to two right angles?’) does not seek what we would call a cause (in Aristotle’s division, an efficient cause). When explanations of changes are being sought, however, Aristotle seems to provide recognizably causal explanations. Even the aitia (material, formal, final) that do not initially seem to be causes turn out to play an important role in causal explanation; for this reason, the label ‘four causes’ gives a reasonably accurate impression of Aristotle’s doctrine.
His comparison between artefacts and natural organisms clarifies his claims about formal and final causes. The definition of an artefact requires reference to the goal and the intended function. A hammer’s form and essence is a capacity to hammer nails into wood. The hammer was designed to have this capacity for performing this function; and if this had not been its function, it would not have been made in the way it was, to have the properties it has. The form includes the final cause, by specifying the functions that explain why the hammer is made as it is.
Similarly, Aristotle claims, a natural organism has a formal cause specifying the function that is the final cause of the organism. The parts of an organism seem to perform functions that benefit the whole (the heart pumps blood, the senses convey useful information). Aristotle claims that organs have final causes; they exist in order to carry out the beneficial functions they actually carry out. The form of an organism is determined by the pattern of activity that contains the final causes of its different vital processes. Hence Aristotle believes that form as well as matter plays a causal role in natural organisms.
To claim that a heart is for pumping blood to benefit the organism is to claim that there is some causal connection between the benefit to the organism and the processes that constitute the heart’s pumping blood. Aristotle makes this causal claim without saying why it is true. He does not say, for instance, either (1) that organisms are the products of intelligent design (as Plato and the Stoics believe), or (2) that they are the outcome of a process of evolution.
Aristotle’s account of causation and explanation is expressed in the content and argument of many of his biological works (including those connected with psychology). In the Parts of Animals and Generation of Animals for instance, he examines the behaviour and structure of organisms and their parts both to find the final causes and to describe the material and efficient basis of the goal-direction that he finds in nature (Parts of Animals I 1). He often argues that different physiological processes in different animals have the same final cause.
Some ascribe to Aristotle an ‘incompatibilist’ view of the relation between final causes and the underlying material and efficient causes. Incompatibilists concede that every goal-directed process (state, event) requires some material process (as nutrition, for example, requires the various processes involved in digesting food), but they argue that the goal-directed process cannot be wholly constituted by any material process or processes; any process wholly constituted by material processes is (according to the incompatibilist) fully explicable in material-efficient terms, and therefore has no final cause.
Probably, however, Aristotle takes a ‘compatibilist’ view. He seems to believe that even if every goal-directed process were wholly constituted by material processes, each of which can be explained in material-efficient terms, the final-causal explanation would still be the only adequate explanation of the process as a whole. According to this view, final causes are irreducible to material-efficient causes, because the explanations given by final causes cannot be replaced by equally good explanations referring only to these other causes. This irreducibility, however, does not require the denial of material constitution.
Aristotle studies nature as an internal principle of change and stability; and so he examines the different types of change (or ‘motion’; kinesis) that are found in the natural elements and in the natural organisms composed of them. In Physics III 1 he defines change as ‘the actuality of the potential qua potential’. His definition marks the importance of his views on potentiality (or ‘capacity’; dynamis) and actuality (or ‘realization’; energeia or entelecheia) (see Metaphysics IX 1-9).
The primary type of potentiality is a principle (arche) of change and stability. If x has the potentiality F for G, then (1) G is the actuality of F, and (2) x has F because G is the actuality of F. Marathon runners, for instance, have the potentiality to run 26 miles because they have been trained to run this distance; hearts have the capacity to pump blood because this is the function that explains the character of hearts. In these cases, potentialities correspond to final causes.
Potentiality and possibility do not, therefore, imply each other. (1) Not everything that is possible for x realizes a potentiality of x. Perhaps it is possible for us to speak words of Italian (because we recall them from an opera) without having a potentiality to speak Italian (if we have not learnt Italian). (2) Not everything that x is capable of is possible for x; some creatures would still have a potentiality to swim even if their environment lost all its water.
These points about potentiality help to clarify Aristotle’s definition of change. The building of a house is a change because it is the actuality of what is potentially built in so far as it is potentially built. ‘What is potentially built’ refers to the bricks (and so on). The completed house is their complete actuality, and when it is reached, their potentiality to be built is lost. The process of building is their actuality in so far as they are potentially built. ‘In so far… ’ picks out the incomplete actuality that is present only as long as the potentiality to be built (lost in the completed house) is still present. Aristotle’s definition picks out the kind of actuality that is to be identified with change, by appealing to some prior understanding of potentiality and actuality, which in turn rests on an understanding of final causation.
In the rest of the Physics, Aristotle explores different properties of change in relation to place and time. He discusses infinity and continuity at length, arguing that both change and time are infinitely divisible. He tries to show that the relevant type of infinity can be defined by reference to potentiality, so as to avoid self-contradiction, paradox or metaphysical extravagance. In his view, infinite divisibility requires a series that can always be continued, but does not require the actual existence of an infinitely long series. Once again, the reference to potentiality (in ‘can always… ’) has a crucial explanatory role.