A traditional metaphysical view holds that substances are fundamental entities that bear or possess properties. Substances persist through qualitative change while their properties vary. This classical framework has been challenged by bundle theory and trope theory but remains influential.
Compare the substance view with bundle theory and trope theory. Consider what it means for a substance to 'underlie' or 'support' properties. Examine whether we need substances as fundamental entities.
That substances are unknowable or mysterious underlying things. That substance view makes properties less real. That substances must be simple or indivisible.
From your study of substance and property, you know the basic distinction: substances are the fundamental individuals — things that exist in their own right — while properties are how those substances are qualified or characterized. From properties intrinsic versus extrinsic, you know that some properties are had by objects independently of how the world around them is arranged, while others are relational. The substance-as-bearer view deepens the first distinction by asking a structural question: what exactly is the relationship between a substance and the properties it has?
The core picture is this: there is an underlying subject — the substance — and there are properties that inhere in or are borne by that subject. The apple is red, round, and tart: redness, roundness, and tartness are properties that the apple possesses, but they are not themselves the apple. The apple is what has them. This seems intuitive enough. But notice the implied structure: there must be something that does the having — some entity that underlies the properties and holds them together into a single individual. This underlying subject is sometimes called the substratum or bare particular.
The philosophical pressure on this view comes from asking what the substratum is like when we mentally strip away all the properties. If we abstract redness, roundness, and tartness from the apple, what is left? It appears to be an entity with no characterizing properties of its own — a "bare" particular that is the subject of predication but has no qualitative nature in itself. Critics (especially bundle theorists) argue this is either mysterious or incoherent: how can something exist with no properties at all? How do we identify or individuate bare particulars from one another if they have no qualitative differences? This is the origin of the Common Misconception that substances are unknowable — it was Locke's own worry about "something I know not what" underlying observed qualities.
The bundle theory offers the alternative: there is no underlying substratum; a substance just is a bundle or cluster of compresent properties. The apple is nothing over and above the co-instantiated cluster of redness, roundness, tartness, and so on. This avoids the mystery of bare particulars but creates its own problems. Most famously, if a substance is just its bundle of properties, then two distinct substances with exactly the same properties would be identical — violating the intuition that numerical identity and qualitative identity come apart. A universe containing two qualitatively identical iron spheres would seem to contain two objects, but bundle theory says they are the same bundle and therefore the same object.
Trope theory offers a third option: instead of universal properties shared across objects, each property instance is a unique, unrepeatable trope — this particular redness of this apple, numerically distinct from any other redness. A substance can then be analyzed as a bundle of tropes rather than universals, which solves some identity problems. The debate among substance-bearer theory, bundle theory, and trope theory is one of the live fronts of contemporary metaphysics, and your study of dispositional versus categorical properties will reveal how the choice among these frameworks ramifies into questions about causation and natural law.
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