Ontic vs epistemic

In plain terms

Quantum mechanics keeps two kinds of statement in one notation. The sentence "the atom is here" sounds like a fact about the world, and the sentence "the atom has a thirty percent chance of being here" sounds like a fact about what you know. The wavefunction, the mathematical object that encodes everything quantum mechanics says about a system, carries both voices at once, and a century of argument has been about which voice is fundamental.

The distinction between map and territory is old, and it only starts to bite when you enforce it. A fact is ontic when it belongs to the territory, so it's true whether or not anyone looks, and a fact is epistemic when it belongs to the map some agent or apparatus draws. The trouble starts when a map-fact is mistaken for a territory-fact, that is, when a change in the description is read as a change in the world.

Collapse, the abrupt jump of the quantum state at measurement, is the canonical casualty. If the state is a map, its sudden update on measurement is no stranger than a weather forecast changing when you open the curtains.

In CSD

CSD makes the split load-bearing and never lets it blur. The outcome regions a measurement context defines live on the projected state space, the space of quantum states, and they're epistemic, as the apparatus fixes the partition and the partition is a description. The measure that weights those regions is ontic, an invariant volume on the constraint surface itself, one the symmetry leaves unchanged, fixed by that symmetry and indifferent to anyone's knowledge.

The Born rule, the quantum rule for outcome probabilities, is precisely the handshake, an epistemic region weighed by an ontic volume. Measurement is the ontic selection, the record written on the surface, of which epistemic region the one real trajectory realises. Keeping the two levels distinct isn't philosophical hygiene, as it's the structure of the reconstruction.

Mathematically

The two-strata reading is written into the axiom ledger. The operational stratum speaks the effect-and-density language and carries the Gleason-type representation, and the ontic stratum computes weights as ratios of the Liouville measure over the preparation region.

The corpus proves that the two strata meet, as the same Born vector is reached by the operational route and by the volume route, through structurally distinct machinery.

Related entries

• Psi-epistemicThe quantum state is what you know and not what exists, and the hard part was always saying what it's knowledge of, so CSD's whole architecture is an answer to that.
• 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.
• 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.
• Constraint surfaceThere's one arena, it's deterministic, and it doesn't grow, and everything else in the programme is bookkeeping about how much of it a measurement context lets you see.
• Many-to-one projectionBehind every quantum state stands a crowd of world-states, and the projection that maps the crowd to one state loses information on purpose, and that loss is where quantum probability comes from.