Substance

The world appears to contain things that have properties: a tomato that is red, a person who is tall. The problem of universals concerned the second term of that relation. This page concerns the first. What is it to be a thing that has properties rather than a property that is had?

"Substance" is the traditional name for the answer, and it carries three distinct ideas that the tradition rarely separated.

Three conceptions

Substance as independent existence. A substance is what can exist on its own; everything else exists only as a modification of, or dependent on, something else. This is the criterion Descartes states and Spinoza takes seriously.

Substance as subject of predication. A substance is what things are said of, and which is not itself said of anything. This is Aristotle's criterion in the Categories, and it is a grammatical test with metaphysical intent.

Substance as bearer and unifier. A substance is what holds a collection of properties together and makes them properties of one thing rather than a scattered plurality.

These come apart, and much of the debate consists of playing one against another. The bearer conception does not require independence — a mode can bear its own features. The independence conception, pressed hard, has the consequence Spinoza drew.

Aristotle

Aristotle distinguishes primary substance — this man, this horse, the individual — from secondary substance, the species and genera under which individuals fall. Primary substances are basic: everything else is said of them or present in them, and if primary substances did not exist nothing else could.

In the Metaphysics the account deepens into hylomorphism: a substance is a compound of matter and form, where the form is what makes it the kind of thing it is and accounts for its unity and persistence. The bronze is the matter of the statue, the shape its form; the flesh and bone are the matter of the animal, the soul its form.

Hylomorphism is an attempt to answer the unity question without a bare bearer: what unifies the properties is not a featureless substratum but a principle of organisation. Whether that is an explanation or a name for the thing to be explained is the standing complaint, and it is the same complaint made against every answer to Bradley's regress.

The early moderns

Descartes adopts the independence criterion and immediately finds it too strong: strictly, only God depends on nothing, so only God is a substance. He therefore introduces a secondary sense in which created substances depend only on God, and there are two of them — res extensa and res cogitans, distinguished by their principal attributes, extension and thought. The mind–body problem follows directly from the category scheme, which is a good illustration of how much such a scheme decides.

Spinoza takes the independence criterion literally and follows it to monism. If a substance is what exists in itself and is conceived through itself, and if two substances could not interact or limit one another, there can be only one: Deus sive Natura. Minds and bodies are not substances but modes — ways the one substance is. This is the cleanest demonstration that the criterion of substance drives the whole ontology.

Leibniz goes the opposite way. True substances must be genuinely unified, and anything extended is divisible and so an aggregate rather than a true unity. Real substances must therefore be simple: the monads, unextended, indivisible, "windowless," each a perspective on the whole. The route from a unity requirement to a plurality of simples is worth noting, since it is the same move that drives mereological nihilism toward simples.

Locke is the sceptic. If we strip away all the qualities of a thing, what remains as their bearer is "something, we know not what" — a supposition we cannot avoid making and cannot give content to. Hume takes the next step: if the idea of substance is not derived from any impression, it is empty, and a thing is nothing but a bundle of its qualities. That is the origin of bundle theory.

Substance now

Contemporary defenders of substance ontology, notably Lowe, argue that objects are an irreducible category and that attempts to construct them from properties fail. Their case rests on three claims: that objects have determinate identity conditions which bundles of properties do not; that objects persist through change of properties in a way a bundle cannot, since a bundle with different members is a different bundle; and that the sortal kind to which an object belongs is what supplies its persistence conditions, not any property it happens to have.

Armstrong's distinction between the thin and the thick particular is a useful piece of bookkeeping here. The thick particular is the object taken with all its properties; the thin particular is the object considered in abstraction from them — the bare bearer. Armstrong holds that these are not two things but one thing considered in two ways, which is a way of accepting the substratum theorist's point without accepting a further entity.

The rivals are the eliminativist positions the section has already met: trope bundle theory, which builds objects from particularised properties; mereological nihilism, which denies composite objects; and process ontologies, which take the basic items to be occurrences rather than continuants.

Where substance sits

This is a category question of the most basic kind: whether the inventory needs a category of bearers alongside the properties borne, or whether one of them can be constructed from the other. It is the counterpart of the problem of universals, and the two are usually settled together — a one-category ontology must eliminate one side or the other.

Its significance for what follows is that the answer determines what an identity question can even mean. If objects are bundles, questions of individuation and persistence are questions about which properties are bundled with which; if they are substances, they are questions about the bearer. The next page takes that fork directly: bundle theory versus substratum.