Psi-epistemic

In plain terms

Two readings of the quantum state have fought for a century. On the realist reading the state is a physical thing, so its jump on measurement is a physical process that needs an explanation the theory doesn't give. On the epistemic reading the state is a description of what someone knows, so the jump is just an update, as you learn something and revise, and it's no more mysterious than a poker player recalculating after seeing a card.

The epistemic reading dissolves the measurement problem, which is its great charm, and its great embarrassment is the follow-up question of what it's knowledge of. If there's nothing underneath, the reading says the theory is about nothing in particular, and a family of more recent theorems makes naive versions of the underneath very hard to hold. The PBR theorem, the most famous of them, shows that if separately prepared systems have independent underlying states, then two different quantum states can never be descriptions of the same underlying state.

So the position is attractive, constrained, and incomplete without a floor.

In CSD

CSD is psi-epistemic with a floor. The state is knowledge of which region of the constraint surface the microstate occupies, so there's a fact underneath, one point following one trajectory, and the state summarises what a preparation pins down about it.

Everything in the programme sorts into two levels, and keeping them apart is the discipline the whole account depends on. The epistemic level holds the space of quantum states, the outcome regions and the probabilities, and the ontic level holds the substrate and the measure that weights it. The corpus's fibre analysis sharpens the split. It treats the space of quantum states as a lossy projection of the substrate, with many microstates mapping to each state, and it places the record- forming content in the fibre above a state, the set of microstates that project to it, rather than on the space of states itself. That placement is a judgement against a tight constraint chain and not a proved dichotomy, as whether the content could stay on the base in three or more dimensions is open in both directions.

Three claims need to be kept apart, because the corpus once ran them together. The first is CSD epistemicity, that the quantum state doesn't completely specify the microstate, because the projection is many-to-one, and that's this entry's subject. The second is the technical Harrigan-Spekkens classification, which calls a model psi-ontic when distinct quantum states never share an underlying microstate and psi- epistemic when they can. On the canonical exact sharp interface, where a preparation is an exact pure state, CSD is psi-ontic, because distinct exact pure states have mutually singular ontic measures, so the space can be split into two parts with each state's measure living entirely on its own part. The third is finite-resolution overlap, as preparations that fix a region of positive volume, rather than an exact state, and whose regions overlap have conditional laws that provably can't be separated onto disjoint supports.

The third claim is the theorem this entry is anchored to. It doesn't establish the second in the epistemic direction, because region preparations are a different preparation class from exact pure states, and describing the overlap theorem as technical psi-epistemicity of exact states was an error that has since been corrected. The first and third claims hold, and on the second the exact interface is psi-ontic.

Mathematically

The overlap statement is proved as conditional_not_mutuallySingular, so overlapping preparation regions give conditional laws that aren't mutually singular. The proof is a density argument rather than a shared-support one, because two measures can both charge a common set and still be mutually singular, so overlap alone wouldn't settle it. The concrete witness kahler_preparations_overlap instantiates it on open neighbourhoods of any two rays of the working arena.

The complementary facts are proved too. Conditioning on an exact pure state is an unreachable idealisation, as single fibres are Liouville-null, what a physical preparation projects to is a density against the Fubini-Study measure and not a point, and distinct exact pure states have mutually singular ontic measures (RecordLayer.sharp_preparations_mutuallySingular), which is the psi-ontic verdict on the exact sharp interface under the Harrigan-Spekkens classification. An earlier version of this entry described the overlap theorem as technical psi-epistemicity of exact states, and that error was corrected on 2026-08-25.

Which literature constraints bite, PBR and its relatives, still depends delicately on what the state is taken to be knowledge of, and on independence assumptions. The full mapping of that boundary isn't formalised, so treat it as live terrain.

Related entries

• Ontic vs epistemicWhat exists is one question and what a measurement context lets you say is another, and most quantum paradoxes are category errors between the two.
• Epistemic measureCondition on what a preparation actually fixes, and the ignorance left over is a measure, and that measure is a theorem rather than a stipulation, as the arena's own volume disintegrates into exactly it.
• Does the PBR theorem rule out CSD?No. For exactly prepared pure states, CSD satisfies the very conclusion the PBR theorem argues for, as two distinct states correspond to sets of underlying world- states that don't overlap at all, and that's a proved theorem rather than something arranged by hand.
• Is the wavefunction real in CSD?The wavefunction is a genuine but incomplete coordinate of a real underlying world, so it can be read off the world's actual configuration, yet it leaves out the part of that configuration which decides what a measurement will show.
• Hidden variablesThe no-go theorems don't kill hidden variables, as they price them, and CSD reads the price tag and pays it.