We will indicate here that basic quantum mechanics does not conflict with our perception of a seemingly classical world, where by ‘basic quantum mechanics,’ we mean the rules in principle [P4] of Basic Quantum Mechanics, QMA —no particles, no collapse, no sentient beings, just the linear Schrödinger equation for the wave function with all its versions of reality.

We already have two relevant principles from the Schrödinger’s Cat section:

[P2] The wave function contains many versions of reality, with a different version of the observer in each one. Only versions of the observer perceive. No one version is singled out as being ‘special’, so quantum mechanics does not predict which version we will perceive.

[P3] In every case where both the calculations and observations can be carried out, our perceptions agree exactly—both qualitatively and quantitatively—with those of one version of the observer.

Principle [P2] is a statement about the mathematical properties of states while principle [P3] tells us that quantum mechanics is never wrong in the sense that the right answer is always contained in the wave function.

Further, from the Separate Universes in Quantum Mechanics section, we have

[P5] Different versions of reality in the wave function are in separate, isolated universes that cannot communicate with or affect each other in any way. Thus no version of the observer can perceive anything other than that which happens within its own, single universe.

so each version of the observer perceives only her own version of reality.

Perception of a single version of reality. Many physicists assume (without a detailed look) that since there are many versions of reality, quantum mechanics, by itself, implies that many versions of reality would be perceived. (And so single-version amendments to quantum mechanics, such as particles or collapse have been proposed.) But that is not correct; even though there are indeed many versions, only one will be perceived, as is shown in Details of Classical Perception.

[P6] Basic quantum mechanics prohibits perception of more than one version of reality.

Agreement among observers. Next, in addition to a single observer perceiving only one version of reality, we also know experientially that two observers never disagree on what they perceive. But this also follows from quantum mechanics.

[P7] Quantum mechanics implies that two observers can never disagree on what they perceive.

Consistency of Perception. Finally if we look in the box twice after doing the Schrödinger’s cat experiment, we certainly expect, from experience, to see the same thing both times. One can show that this holds in quantum mechanics also.

[P8] If two observations are made successively, quantum mechanics implies the same consistency of results as one obtains in a classical universe. A technical version of this principle is that if one measures the same property twice in a row, quantum mechanics implies one will get the same result.

These principles show that each version of the observer perceives a world that exactly matches our ‘classical’ perceptions (of a seemingly objective, single-version world). And since nothing perceives in QMA except the versions (there are no ‘sentient beings’ that perceive more than one version), this shows that QMA does not lead to any perceptions in conflict with our classical expectations.

The justifications of these principles, along with a comment on superselection rules, the ‘preferred basis’ problem, and ‘measurement theory’ are given in Details of Classical Perception.