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Grounding is a relation of metaphysical determination: when we say the mental is grounded in the physical, or the moral in the natural, we mean that the former holds in virtue of the latter. Kit Fine, Gideon Rosen, and others have argued that grounding is a distinctive relation — irreflexive, asymmetric, transitive — that carves out a hierarchy of metaphysical priority distinct from both causation and supervenience. Fundamentality is the bottom of this hierarchy: the fundamental facts are those not grounded in anything further. The grounding framework promises to unify disparate debates — physicalism, mathematical ontology, normativity — under a single structural question: what is prior to what? Critics question whether grounding is a single unified relation or a grab-bag of heterogeneous dependencies.
Read Fine's 'Guide to Ground' for the formal framework, then Schaffer's 'On What Grounds What' for the picture of metaphysics as a discipline studying grounding structure rather than existence. Compare grounding with supervenience and ask what explanatory work grounding does that supervenience cannot.
You come to this topic with a grasp of ontological categories — what kinds of things exist — and likely some exposure to truthmakers and logical consequence. Grounding asks a different question from any of these: not "what exists?" but "what is prior to what?" or "what explains what, in a non-causal, constitutive sense?" This is the relation Kit Fine called metaphysical determination: the fact that a whole is composed of parts doesn't cause the whole to exist in a temporal sense, but the parts' arrangement *grounds* the whole's existence — it's in virtue of the parts being arranged this way that the whole is what it is.
The formal features of grounding help clarify what it is. The relation is irreflexive (nothing grounds itself), asymmetric (if A grounds B, then B does not ground A), and transitive (if A grounds B and B grounds C, then A grounds C). This gives us a strict partial ordering — a hierarchy running from grounded facts up to fundamental ones. These features distinguish grounding sharply from causation, which can be symmetric (mutual causation) and certainly doesn't ground — the fire that causes the smoke doesn't constitute the smoke in the way that the molecular arrangement of water constitutes its liquidity. Grounding is always synchronic and constitutive; causation is diachronic and productive.
Why introduce grounding at all? Consider the physicalist claim that mental facts hold "in virtue of" physical facts. Supervenience captures a correlation — mental facts can't differ without physical facts differing — but it says nothing about explanation or priority. You could have systematic supervenience without any explanatory relation: maybe mental and physical facts co-vary because both depend on something third. Grounding asserts the stronger claim: the physical facts are explanatorily prior; the mental facts obtain because of them. Similarly for moral facts and natural facts: the Humean might say that an act being wrong is grounded in its causing unnecessary suffering, not merely that the two co-vary. Grounding gives structure to these "in virtue of" claims that supervenience alone cannot provide.
Fundamentality is the limiting case: a fact is fundamental if it is not grounded in anything further. Most metaphysicians locate fundamentality in physics, though this is contested — perhaps mathematical or logical facts are fundamental in a different sense, or perhaps there are multiple independent grounding hierarchies. The framework promises to unify disparate philosophical debates under one structural question: whatever you think is real, how is it ordered? What rests on what? The main challenge critics raise is whether "grounding" names a single unified relation or a family of loosely related explanatory relations — metaphysical, causal, logical, conceptual — that don't really share the formal properties claimed. Engaging with this challenge requires knowing the formal framework well enough to test it against cases.
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