Aristotle: From Substance to the Four Causes

This is the third article in the series Understanding Philosophy: Thinkers, Questions and Ideas. The previous article examined Plato’s use of Forms to account for the stability of knowledge. This article turns to Aristotle, who brings form back into particular things and uses substance and the four causes to explain what things are, how they change and why they take the shape they do.

Aristotle’s philosophy can initially appear to be a dense landscape of classifications and technical terms. Behind those distinctions, however, lies a persistent and quite ordinary question: what do we understand when we genuinely understand a thing? Is it enough to know the materials of which it is composed? Have we explained it once we identify the person or event that produced it? If it changes continuously, on what basis do we say that it remains the same thing? Substance, form, matter, potentiality, actuality and the four causes all develop around these questions.

Aristotle studied for many years in Plato’s Academy, but he did not accept the separation of Forms from particular things. Plato wanted unchanging Forms to explain how stable knowledge could exist. Aristotle worried that a form located on another level of reality could not adequately explain how the things before us arise, move and perish. To say that a particular horse is a horse because it participates in the Form of Horse seems to move the original question to a second object rather than answer it. Aristotle therefore looks for intelligible structure within concrete beings themselves.

Substance stands at the centre of this attempt. In the most immediate sense, a substance is something capable of existing as a particular being: this person, this tree or this horse. Colours, sizes, positions and conditions belong to something else. Whiteness is always the whiteness of some object; sitting is something that a particular person or animal is doing. Substance is not simply a lump of physical material. It is the underlying unity to which qualities and changes can belong.

Within concrete substances, Aristotle distinguishes matter from form. The matter of a bronze statue is bronze, while its form is the structure and organisation that makes this bronze a statue of a certain kind. Bronze alone cannot explain why the object is a statue rather than an unworked mass. Form alone cannot explain how that structure becomes a tangible individual in the world. Concrete things are generally composites of matter and form. Aristotle does not remove form from philosophy; he rejects the idea that form must exist separately from the things it makes intelligible.

The relation between matter and form also makes change understandable. An acorn is not yet an oak tree, but it is not an oak in the same way that it is not a horse or a stone. It has the potential to become an oak. Under appropriate conditions, that potential can be actualised. The distinction between potentiality and actuality avoids two extremes. Change need not be the production of something from absolute nothing, but neither must every possibility already be hidden inside a thing in a completed state. Change is the actualisation, through suitable conditions and activity, of a determinate capacity belonging to a being.

On this basis Aristotle develops his famous account of the four causes. The Greek notion translated as “cause” is broader than the meaning the word usually carries today. Modern speakers often think of a cause as an earlier event that produces a later event. Aristotle asks a more extensive question: what different kinds of answer are required for an adequate explanation? He distinguishes material, formal, efficient and final causes. They are not four rival events from which we must choose, but four dimensions through which one thing may be understood.

The material cause answers what a thing is made from. A house is constructed from bricks, timber, glass and other materials; a bronze statue has bronze as its matter. Materials provide real conditions and constrain the ways in which an object can exist. Timber and stone have different properties, permit different structures and bear forces differently. Yet even a complete inventory of materials does not explain why these materials constitute a house rather than a pile of building waste.

The formal cause answers what the thing is and why its parts are organised in this relation. The structure, proportion and arrangement of a house make its materials into a recognisable and usable dwelling. In living beings, form is not merely a geometrical outline imposed from outside. It is the organising principle of living activity. An animal is a unified organism not only because particular chemicals are present in its body, but because its parts act together in ways that sustain perception, movement, nutrition and reproduction.

The efficient cause identifies the source from which change begins, or what brings a process of formation into actuality. A builder is an efficient cause of a house. A sculptor makes bronze take on the form of a statue. Parents have an efficient role in the generation of living beings. Mechanical causation in modern science most closely resembles this dimension, though the two are not entirely identical. Aristotle is not merely searching for the first push in a chain; he is identifying the source through which a determinate change comes about.

The final cause answers what something is for, or towards what completion its activity is directed. A house is built for habitation, and the structure of a tool is related to the work it is intended to perform. In natural things, purpose does not necessarily imply that an external designer has prepared a conscious plan. The activity of an eye is directed towards seeing; the development of a seed is directed towards a mature living form. Aristotle’s natural teleology first of all explains development and activity through the characteristic completion of the being concerned.

The four causes are easy to distinguish in the example of a house, but they frequently converge in living nature. The form of the mature organism is also the end towards which development proceeds. An organism of the same species is normally an efficient source in the generation of a new individual. For Aristotle, this overlap is not conceptual confusion. It expresses the internal organisation of life. A living being is not a heap of components assembled by accident. It actively maintains itself, repairs certain kinds of damage and grows along a path characteristic of its kind.

The four causes are therefore not a demand that every explanation recite four items mechanically. They are a warning against mistaking one dimension for a complete account. Knowing that a book consists of paper and ink is not the same as understanding its argument. Discovering that neural activity accompanies a behaviour does not by itself explain what the behaviour means to the person acting. Identifying the historical events that produced an institution may still leave its function and aims unexplained. Different objects call for different levels of account, and fuller understanding often requires connecting those levels.

The development of modern science brought strong objections to final causes. Physics can explain the movement of bodies without supposing that a stone “wants” to reach a place. Evolutionary theory can account for biological functions through natural selection without appealing to a plan established in advance. When every natural process is personified, the language of purpose can obstruct investigation. It does not follow, however, that function and purpose have become meaningless. An explanation of the heart’s structure still refers to the function of pumping blood. In the study of action and institutions, reasons and goals cannot simply be replaced by descriptions of material motion.

Aristotle’s importance lies in his refusal either to reduce reality to unstructured matter or to place form in a world remote from experience. Things are intelligible because they have materials, organisation, processes of generation and characteristic directions of activity. A substance is not necessarily a static and unchanging block. It is a being that can preserve a kind of unity through change and actualise its capacities in determinate forms of activity.

From substance to the four causes, Aristotle establishes a habit of layered explanation. Faced with an object, we can continue to ask: what is it made from? What structure makes it the kind of thing it is? What process brought it into existence? Towards what completion is its activity directed? These questions do not guarantee that every object has a simple answer. They prevent us from prematurely treating one kind of account as the whole truth. To understand a thing is not only to locate the event immediately preceding it. It is to see how that thing, as a whole, comes to be what it is.