When a statue is sad, what makes it sad? Not causation—nothing caused the statue's sadness the way heat causes water to boil. The sadness seems to arise from the arrangement of bronze, the curve of lips, the angle of eyes. But how do we capture this relation of metaphysical dependence?

For decades, philosophers relied on supervenience—the idea that certain facts necessarily covary with others. Mental states supervene on brain states; aesthetic properties supervene on physical arrangements. Yet supervenience merely describes patterns of covariation. It tells us that things go together, not why.

Enter grounding: a relation of metaphysical explanation that has transformed how contemporary philosophers understand reality's hierarchical structure. Grounding answers the question supervenience leaves hanging—what makes what the case? This conceptual innovation provides tools for understanding how facts at one level of description relate to facts at another, and ultimately, what counts as metaphysically fundamental.

Supervenience captured something important: certain facts don't float free from others. You can't have two worlds identical in physical facts but different in mental facts (assuming physicalism). But consider a revealing asymmetry: mental facts supervene on physical facts, yet physical facts also supervene on mental facts, trivially. Any difference in physical facts yields a difference in mental facts too.

This symmetry reveals supervenience's blindness to explanatory direction. When we say the mental depends on the physical, we mean something asymmetric: the physical explains the mental, not vice versa. The brain state makes it the case that you're in pain; your being in pain doesn't make it the case that your C-fibers are firing. Supervenience cannot capture this directionality.

Consider another problem. Necessarily, if Socrates exists, then {Socrates}—the set containing only Socrates—exists. And necessarily, if {Socrates} exists, Socrates exists. Perfect supervenience in both directions. Yet clearly Socrates's existence explains the set's existence, not the reverse. The set exists because Socrates exists; Socrates doesn't exist because the set does.

Grounding fills this explanatory gap. When we say that Socrates's existence grounds the existence of {Socrates}, we assert a relation of metaphysical determination. The grounded fact obtains in virtue of the grounding fact. This 'in virtue of' locution tracks explanatory priority in a way mere necessary covariation cannot. Grounding is supervenience with an arrow.

Takeaway

Supervenience tells us facts covary necessarily; grounding tells us which facts make which facts the case. Pattern recognition differs from explanation.

What distinguishes grounding from other philosophical relations? Three formal properties prove crucial: irreflexivity, asymmetry, and transitivity. Nothing grounds itself—a fact cannot explain its own obtaining. If A grounds B, then B does not ground A—explanatory priority flows in one direction. And if A grounds B and B grounds C, then A grounds C—chains of metaphysical explanation compose.

These features constrain theorizing in powerful ways. Irreflexivity rules out self-explanatory facts at the fundamental level; something explains each non-fundamental fact, but the explanation cannot be circular. Asymmetry ensures that grounding chains have direction—we can meaningfully ask what's more fundamental. Transitivity means distant groundings connect: if neural facts ground mental facts and mental facts ground social facts, neural facts ground social facts.

Contrast this with causation. Causation appears asymmetric and transitive, but its irreflexivity is debated—some argue backward causation allows causal loops. More importantly, grounding is synchronic where causation is typically diachronic. The statue's sadness doesn't come after the bronze arrangement—it obtains at the same time, in virtue of it. Grounding structures reality at a moment; causation structures it across time.

Some philosophers distinguish full from partial grounding. A disjunction (P or Q) is fully grounded by P alone, or by Q alone. But a conjunction (P and Q) is only partially grounded by each conjunct—you need both for full grounding. This distinction enables precise analysis of how complex facts decompose into their metaphysical sources.

Takeaway

Grounding's irreflexivity, asymmetry, and transitivity make it a strict partial ordering on facts—a backbone of hierarchical structure running through reality.

What does it mean for something to be metaphysically fundamental? Grounding provides a crisp answer: fundamental facts are ungrounded facts. They don't obtain in virtue of anything else. They sit at the bottom of the grounding hierarchy, explaining without being explained. Everything else—derivative facts—owes its obtaining to the fundamental.

This definition sharpens otherwise vague debates. Is consciousness fundamental? Ask whether conscious facts are grounded in non-conscious facts. Are numbers fundamental? Ask whether mathematical facts hold in virtue of non-mathematical facts. The grounding framework doesn't settle these questions, but it makes them precise. We know exactly what we're disputing.

The framework also illuminates the structure of fundamentality. Perhaps fundamentality comes in degrees—facts closer to the ungrounded base are more fundamental than those higher up. Or perhaps there's a sharp cut between fundamental and derivative. Either way, grounding provides the metric. We measure distance from the fundamental by counting grounding steps.

Consider physicalism reformulated: all facts are either physical facts or grounded (ultimately) in physical facts. This says physical facts are fundamental; everything else derives from them. Dualism denies this—mental facts are fundamental, ungrounded in the physical. The grounding idiom transforms traditional metaphysical positions into precise claims about what grounds what, enabling clearer assessment of what's at stake and what evidence bears on these disputes.

Takeaway

Fundamentality isn't primitive mysteriousness—it's defined structurally as absence of grounds. What's most real is whatever explains without being explained.

The grounding revolution offers metaphysics a conceptual precision it previously lacked. Where supervenience saw only patterns of covariation, grounding sees explanatory structure—the backbone of metaphysical dependence running from fundamental to derivative.

This framework doesn't tell us what's fundamental. It gives us the tools to articulate competing answers and understand what accepting them commits us to. Physicalism, dualism, mathematical platonism—all become precise positions about grounding relations.

Philosophy advances not only through new answers but through better questions. Grounding represents such an advance: a more refined instrument for investigating reality's deepest architecture.