Intrinsic (non-relational) properties belong to objects independently of other entities; relational properties hold between objects or between an object and external entities. Whether all properties reduce to intrinsic properties at a fundamental level, or whether relations are primitive, deeply shapes metaphysical pictures of causation, properties, and fundamental ontology.
From intrinsic and extrinsic properties, you already know the basic contrast: some properties an object has regardless of what else exists (being 5 kg, being spherical), while others an object has only in virtue of how it stands to other things (being heavier than Sam, being north of the river). The distinction between intrinsic (non-relational) and relational properties sharpens this contrast by asking specifically whether a property is *constituted* by a relation to something external, or whether it belongs to the object considered entirely alone.
The distinction shows up clearly in concrete cases. A ball's mass and charge are paradigm intrinsic properties — if you transported the ball to an otherwise empty universe, it would keep its mass and charge unchanged. But "being the most massive object in the room" is relational: it can be gained or lost as other objects enter or leave the room, without anything about the ball changing internally. More subtly, "being warmer than the planet Jupiter" is relational — it depends not just on the ball's temperature but on Jupiter's. Temperature itself, though, is typically treated as intrinsic: the ball has a particular temperature independently of what else exists.
The philosophically significant question is whether relations are reducible to intrinsic properties. Some positions in metaphysics, particularly those influenced by Leibniz, hold that all genuine facts ultimately reduce to facts about the intrinsic properties of individual substances — relational truths are just a way of describing how intrinsic properties compare. On this view, "A is larger than B" reduces to: A has intrinsic size S1, B has intrinsic size S2, and S1 > S2. No irreducible relation between A and B is needed. Physicists and metaphysicians following a different tradition (including many who take structural realism seriously) argue instead that some relations are primitive — they cannot be reduced to the intrinsic properties of the relata. The spatial relations in quantum entanglement and the web of causal dependencies in physics are frequently cited as examples.
This debate connects directly to causation, which you've encountered. If causation is a relation, is it reducible to the intrinsic properties of the cause and effect — their categorical dispositions — or is it a primitive relational fact? Humeans want to reduce causation to patterns of co-occurrence among intrinsic properties; non-Humeans posit primitive causal relations or necessitation as irreducible. The answer matters for fundamental ontology: a world with only intrinsic properties is structurally different from a world where relations are load-bearing features of reality, not just shorthand for comparing intrinsics.
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