Since there seems to be a bit of confusion on this, I thought I'd just post a brief summary. Just some terminology: Superobserver: Somebody who measures another observer, i.e capable of resolving the complete quantum state of another observer and performing measurements on it. Hyperobserver: Like a Superobserver, but also capable of using a unitary evolution to reverse the state of an observer to its pre-measurement form. (I just made this term up to distinguish the two cases, it's not standard) In essence the Frauchiger-Renner theorem derives a contradiction between: Validity of Probability One predictions of quantum theory, i.e. if QM says something has 100% chance of occurring it is certain. Single World, i.e. experiments have one objective outcome Inter-agent reasoning, i.e. I can obtain my predictions by reasoning about how you would use quantum theory. Intervention insensitivity for Classical Objects/Measurement results. As a superobserver your reasoning about measuring an observer is not affected by subsequent measurements by superobservers spacelike separated from you. In short this says that observers aren't to be considered as being entangled/Bell-inequality violating by superobservers. This is equivalent to the following reformulation by Richard Healey which I think is easier to grasp: Quantum Mechanics applies objectively to all systems/is universal Single World Superobservers should use superposed states to describe observers, prior to their measurements of them Intervention insensitivity Most criticism of the FR paper is because they don't mention (4) as an assumption and thus you can escape dropping the other three assumptions by dropping it. However note that dropping (4) does mean that observers cannot be considered as purely Classical, so a very strict form of Copenhagen is blocked. Also note that the Frauchiger-Renner theorem does not use HyperObservers so it doesn't assume measurements are reversible. There is an alternate form of the theorem due to Luis Masanes, which is in truth a separate theorem which derives a similar contradiction, but replaces (4) with: (4*) It is possible to unitarily reverse a measurement, i.e. HyperObservers exist. Again here you might deny (4*) if you wanted a certain type of Copenhagen interpretation. However since from the point of view of a HyperObserver they are licensed to use superposed states (via (3)) they'd have no reason to suppose some unitaries don't have physically realisable inverses, so this would have to take the form of an ad-hoc restriction of QM when used by such observers. Of course one might deny (3) and (4*), observers shouldn't be modeled with superpositions and you can't reverse measurements. This would be objective collapse like the Ghirardi–Rimini–Weber theory.