Wave function collapse

If the packet is to be reduced, the interaction must have produced knowledge in the brain of the observer. If the observer forgets the result of his observation, or loses his notebook, the packet is not reduced. We are again led to emphasize the fact that the wave function of a pure-state assemblage is merely a mathematical tool for computing from all previous observations what the relative probabilities are for different results when we make our next observation.

— Wave function collapse

To Einstein the collapse postulate was an even more pernicious retreat from realism than that described above: it implied that physical quantities usually have no values until they are observed, and therefore that the observer must be intrinsically involved in the physics being observed.  This suggests that there might be no real world in the absence of an observer!

— Wave function collapse