psi-epistemic vs psi-ontic

The psi-epistemic versus psi-ontic split is a sharpened form of the wavefunction-reality debate, and it has become a precise technical question. A psi-ontic view holds that the quantum state is a real feature of the physical system — knowing it tells you something about the world itself. A psi-epistemic view holds that the state represents only knowledge, or information, about an underlying reality that may be described by other variables.

The psi-epistemic idea is appealing because it would tidy up several puzzles at once. Collapse would be nothing more than updating one's information when a measurement reveals something, just as a probability distribution snaps to a value when you learn the answer; two observers could hold different states without contradiction; and the wavefunction would not need to be a real object in its strange high-dimensional space. For a while this looked like the comfortable, deflationary option.

Then in 2012 the PBR theorem (Pusey, Barrett and Rudolph) changed the terrain. It proved that, under a natural assumption that independently prepared systems have independent physical states, the wavefunction cannot be purely epistemic — it must reflect something real about each system. The honest summary is that fully psi-epistemic models of the simple kind are ruled out, though some elaborate or assumption-dropping versions, and views like QBism that reject the framing entirely, remain in play.