Aristotle's Metaphysics: Being, Substance, and the Unmoved Mover

Imagine a house taken apart and its materials stacked in a yard. The bricks and timber remain, but the house has gone. An inventory of materials therefore leaves something unexplained: what made those materials a house? Aristotle asks a version of this question in Metaphysics VII.17. He wants the cause that explains why some matter constitutes a definite thing. The answer he investigates is form: what makes the materials the kind of whole they are.

This question reaches beyond buildings. Suppose you describe a friend as tall, tired, and a good musician. You have said several things about someone, but those descriptions presuppose a subject whose height, tiredness, and skill they are. Aristotle's question is what has this basic status, and what makes such a thing what it is. The translation’s central term is substance. Here substance means something closer to a basic being than to a chemical material.

The Metaphysics investigates how these questions fit together: what exists primarily, how changing things can be intelligible, and whether reality includes an unchanging source of motion. The inquiry does not offer one short definition that settles everything. We need to follow its distinctions, and notice where a promising answer creates another problem.

Book I.1–3 gives the inquiry its direction. Aristotle begins with the human desire to know. He distinguishes experience of particular cases from understanding the reason why they happen. Someone may know from experience that a treatment helped several patients; the person with the relevant art understands a general explanatory account. The contrast concerns explanatory knowledge, rather than a dismissal of experience.

Wisdom, in this discussion, seeks first principles and causes. Aristotle also presents philosophical inquiry as arising from wonder and being pursued for understanding itself. This matters for the reader expecting immediate advice. The question is initially what would make reality intelligible, rather than how a theory might improve productivity. Practical applications should follow from the distinctions, rather than determine in advance which conclusions are acceptable.

Aristotle introduces four senses of cause in I.3.

• A material cause answers what something is made from.
• A formal cause answers what it is, or gives its defining account.
• A moving cause, often called an efficient cause, identifies the source from which change begins.
• A final cause identifies that for the sake of which something occurs.

Aristotle's four senses of cause include more than a source producing a change.

Consider a deliberately simplified contemporary example: a workshop makes a wooden chair for a classroom. The timber, the arrangement that makes it a chair, the workshop's making of it, and the purpose of seating students answer different questions. None of these answers automatically eliminates the others. This is an illustration of Aristotle's distinctions, not his example or a claim that every artifact is a natural substance. Its point is that asking what caused something requires clarity about which explanatory question is being asked.

Book IV.1 proposes a science of being qua being. Qua means in the capacity of, or insofar as. A specialist inquiry studies things under a restricted aspect; Aristotle's proposed inquiry considers what belongs to things insofar as they are beings. He is not proposing an encyclopedia containing all the findings of every particular discipline. He is asking about principles that bear on being generally.

An immediate difficulty is that being has several senses. A person is, a quantity is, and a quality is, but their modes of being need not be identical. In IV.2 Aristotle compares this pattern to the different ways something can be called healthy. A condition may be healthy because it preserves health, while another thing is called healthy because it indicates health. The meanings are connected through a central reference, rather than being unrelated coincidences of language.

He proposes a corresponding connection among senses of being, with substance as their focal reference. A quality can be a quality of a substance; a process can be a coming-to-be or destruction of substance. This gives the investigation a direction without requiring every being to belong to one undifferentiated category. To understand being generally, Aristotle gives priority to understanding substance.

The distinction can discipline an ordinary discussion. Suppose a project report gives an average response time without identifying which requests it measures. The number may be perfectly calculated while its application remains unclear. This contemporary analogy illustrates the need to identify the subject of a predication. It does not establish Aristotle's theory of numbers, or prove that every statistic is metaphysically an accident of one substance.

The inquiry also concerns principles used across different subjects. In IV.3, Aristotle formulates noncontradiction: an attribute cannot both belong and fail to belong to the same subject at the same time and in the same respect. The qualifications matter as much as the prohibition. Someone can be tired now and rested tomorrow. A surface can be warm on one side and cold on another without the same part being both in the same respect. Neither example violates the principle he states.

In IV.4 Aristotle denies that every principle can receive a demonstration from more basic premises. An unlimited demand for prior demonstration would prevent demonstration from getting started. Instead, he offers a refutative defense against someone who rejects noncontradiction while still saying something definite. The interlocutor must make a meaningful distinction if a word is to signify something determinate in the discussion. Aristotle tries to show that unrestricted denial undermines the distinctions required for the denial itself.

This is a defense through the conditions of intelligible argument, rather than a proof from a supposedly more certain axiom. One can assess whether each step succeeds without confusing the strategy with a refusal to give reasons. For present purposes, its practical use is modest: specify the subject, time, and respect before declaring two claims inconsistent. That habit can expose a verbal dispute before anyone mistakes it for a discovery about reality.

Book VII.1 renews the inquiry by connecting the question of being to the question of substance. Aristotle presents substance as prior to the other categories in definition, knowledge, and time. He explains the temporal priority through substance’s independent existence, since the other categories cannot exist on their own. This does not mean that a thing first exists without any properties and only later acquires them. His motivating contrast is between what a thing is and such descriptions as its quality or quantity. Knowing that something is pale does not yet tell us what the pale thing is. The basic explanatory subject remains to be identified.

In VII.3 he lists essence, universal, genus, and underlying subject among the candidates for substance. Within the discussion of the underlying subject, he distinguishes matter, form, and their compound. The bronze, the form of a statue, and the concrete statue exemplify those distinctions. These are candidates examined within an inquiry; they are not three interchangeable labels with no further questions attached.

Matter has an obvious attraction: it seems to underlie the changes in a thing's characteristics. But imagine stripping away every determination, including quantity and shape, to isolate a wholly indeterminate underlying subject. Aristotle objects that this result does not satisfy the separability and determinacy associated with substance. An answer that leaves us unable to say what anything is has failed to explain precisely what the investigation seeks.

That argument gives form a claim to explanatory priority. It does not mean that ordinary living things cease to be substances, or that only a detachable abstract form is real. Aristotle explicitly includes the compound of matter and form in the discussion. The interpretive task is to distinguish the concrete thing from what explains its being the definite thing it is. Treating the text as a simple elimination contest, in which matter and compound disappear entirely, obscures that task.

VII.13 introduces a further constraint by arguing against the universal as substance. Aristotle reasons that substance belongs peculiarly to the individual thing, whereas a universal is common to several things. This complicates any easy equation of form with a general property shared by many individuals. A reader has to ask how an explanatory account can apply to many things without itself becoming another independently existing thing alongside them. The book makes this problem visible; a short introduction should not pretend to resolve every tension it generates.

VII.17 offers a particularly useful way into the positive proposal. Aristotle treats substance as a principle and cause, and asks why certain materials are a house. The inquiry seeks what makes the matter a definite thing. Form answers that explanatory question. The answer is more informative than a repetition of the inventory of bricks and stones.

Return to the dismantled house. An account of the arrangement and organization of its materials helps explain what was lost when it was taken apart. However, a house is an artifact, and we should be cautious about transferring every feature of its explanation to a living organism. Aristotle's Physics II.1 distinguishes natural things, which have an internal source of change and rest, from artifacts considered as products of an art. The artisan analogy introduces a distinction; it does not erase the difference between growth and construction.

A related account appears in On the Soul II.1. There Aristotle describes the soul as the form or first actuality of a natural organized body capable of life. His analogy in II.1 distinguishes possessing knowledge from exercising it. First actuality here means possessing a living capacity, while its present exercise is a further actuality. Soul in this account means a principle of living activity, rather than simply the outline of a body. Plants count as living through nutrition and growth, so the discussion is broader than a theory of conscious thought alone. This related text helps explain why form should not automatically be translated in one's mind as visible shape.

There is also a challenge to the proposal. If form is introduced merely as the name for whatever makes the materials a whole, have we explained anything? An Aristotelian answer must supply a determinate account of the thing's organization and characteristic activities. Calling a difficult case a matter of form cannot by itself settle what its form is. The distinction earns its explanatory value only when an account shows why these materials belong together as this kind of thing.

Book IX.3 addresses a position attributed to the Megaric school: a thing can act only when it is acting. Aristotle's builder supplies a counterexample. A builder does not lose the art of building whenever the work stops and acquire it again when work resumes. Possessing a capacity and exercising it must therefore be distinguished.

In IX.6 Aristotle explains actuality by analogies, including building compared with being able to build, and being awake compared with being asleep. He does not restrict the distinction to a single model of physical change. Potentiality concerns what something can be or do, while actuality concerns its being or activity as realized. These terms require attention to the relevant capacity and circumstances. They do not turn every imaginable future into a real possibility for the thing under discussion.

For a contemporary illustration, imagine a pianist waiting silently before a performance. The silence alone does not establish that she lacks the skill to play. Conversely, wanting to perform a difficult piece is not sufficient to possess that skill. The distinction helps separate an available capacity from an aspiration and from its present exercise. This example draws a conceptual lesson from Aristotle; it makes no claim about experimental psychology.

In IX.8 Aristotle argues that actuality is prior to potentiality in account and substance, while treating temporal priority with qualifications. We understand a capacity by reference to the activity for which it is a capacity. A particular developing thing may precede its own mature activity in time, yet Aristotle also appeals to something already actual in explaining its development. The claim is therefore more precise than saying that every actual state always occurs earlier than every potential one.

Book XII.6–7 turns to an eternal unmoved substance. Aristotle's starting point here is eternal motion and time, including the everlasting circular motion he attributes to the heavens. He is seeking a principle adequate to that motion. The argument is not about a first event that happened before an otherwise motionless universe.

A merely potential source of motion would be insufficient, he argues, because a capacity need not be exercised. To account for motion that never fails, he requires a principle whose essence is actuality. In his argument, the unmoved mover is eternal and immaterial. These conclusions belong to a framework whose cosmological assumptions must themselves be assessed, rather than treated as observations established by the argument.

How could a source move something without itself undergoing change? XII.7 uses objects of thought and desire to introduce that possibility. An end can be the object toward which an activity is directed without being pushed or altered by that activity. Aristotle describes the first mover as moving in the manner of a final cause, as something loved.

Imagine a musician practicing in order to achieve an excellent performance. The intended performance helps explain why the practice is organized as it is, without acting like a hand physically pushing the musician. This analogy clarifies one kind of explanatory relationship. It does not prove that a celestial body desires an immaterial object, or supply the missing premises of Aristotle's cosmology. The difficult passage needs those limits to remain visible.

In XII.9 Aristotle characterizes divine thinking as thinking of thinking. His discussion seeks the best and most self-sufficient object for divine thought. It should not be casually expanded into a detailed account of providence or a creator's plans for human history. Nor does XII reduce neatly to one mover with no further complication: XII.8 relates additional unmoved substances to additional celestial motions. The primacy of one mover and the plurality discussed there both belong in an account of the text.

A serious objection concerns the move from explanation to an unmoved source. Why must a sufficient explanation have the specific form Aristotle demands? Even someone who grants that particular changes have causes can question the premises about eternal circular motion or the adequacy of final causation for explaining it. Within XII, those premises carry argumentative weight; repeating the conclusion cannot establish them independently.

Another difficulty concerns unity. If matter and form are distinguished, why do they constitute one thing rather than two? In VIII.6, Aristotle responds by treating the relevant matter as potentially what the form is actually. The distinction is not between two independently completed objects requiring a third object to join them. The critic can still ask whether this explains unity or redescribes it. That is a substantive disagreement about what an adequate explanation must provide.

These questions suggest a useful way to read the book now. When investigating a difficult case, separate a list of materials from an account of organization, an available power from its exercise, and a purpose from a source of physical change. Then ask which distinctions help with the particular case and which assumptions remain unproved. No immediate conclusion about machine consciousness, corporate personhood, or personal identity follows simply from using the vocabulary. Those applications would require additional arguments about what the relevant things are.

The house in the yard offers a starting discipline. Before claiming to understand something, ask whether you have named its components, described a property, identified a capacity, or explained why it is the definite thing it is. Aristotle's investigation asks us to distinguish those achievements. Its difficulty is productive when it makes an explanation more exact, rather than furnishing a grand name for what remains unexplained.

• Aristotle, Metaphysics, translated by W. D. Ross. Read I, IV, VII–IX, and XII alongside this introduction.
• Aristotle, Physics, Book II, translated by R. P. Hardie and R. K. Gaye, for nature and explanatory causes.
• Aristotle, On the Soul, Book II, translated by J. A. Smith, for the account of life in terms of form and actuality.