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Sortal concepts like 'horse', 'artifact', and 'person' determine identity conditions: what counts as the same horse or person across time. Different sortals impose different identity criteria—a person persists through mental continuity while matter persists through spatiotemporal continuity. Understanding sortals is essential for addressing persistence, material constitution, and the nature of kinds.
When you ask "Is this the same X?" the answer depends entirely on what X is. A sortal concept is a concept that classifies things into a kind and, crucially, supplies the conditions under which two things at different times count as the same instance of that kind. "Horse," "club," "river," and "person" are sortals; "red" and "heavy" are not (they describe properties but don't individuate). The philosophical point is that identity is not a bare logical relation — it is always identity *as* something, governed by the sortal under which you're tracking the thing.
You've already studied universals and particulars: particular things are individual instances that exist at specific places and times. The sortal question is what makes a particular the *same* particular across time and change. Consider a ship whose planks are gradually replaced one by one. After all original planks are replaced, is it the same ship? Under the sortal artifact, the relevant criterion might be continuity of function and social role — so yes, it's the same ship (the Ship of Theseus). But if the original planks are reassembled elsewhere, which one is the ship? Different theories of artifact identity answer differently. The puzzle is real precisely because sortals are doing the work, and "ship" may not have a perfectly determinate answer.
The contrast between person and body demonstrates how different sortals can apply to the same physical stuff and give different verdicts. Suppose a human being undergoes complete amnesia and psychological discontinuity following brain surgery. Is this the same person? Under psychological continuity criteria for persons (Locke's view: personal identity follows memory and psychological connectedness), perhaps not — the person has ceased and a new one begun in the same body. But under bodily or biological criteria for organisms, it's clearly the same human animal. Neither answer is wrong in isolation: they are answers to different questions governed by different sortals. This is what Wiggins meant by saying identity is always sortal-relative.
The deeper point connects to your prerequisite material on composition and simples. A sortal doesn't just track a thing over time — it determines what counts as a thing at all. "Club" picks out an entity constituted by some members and a set of rules; if those members change completely, the club may still exist because club-identity is constituted by institutional facts, not material ones. "River" picks out a process of water flow; the water molecules at any moment are transient, but the river persists as long as the flow continues in that channel. Each sortal carries a hidden theory of what matters for identity. Making that theory explicit is what sortal analysis does — and it reveals that many identity puzzles (Is this the same organization? Is she still the same person after such profound change?) cannot be resolved without first settling what sortal you're asking about.
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