John Von Neumann:

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When analyzing the evolution of a system, Von Neumann distinguished between processes of the first and second kinds. The latter occur in isolated systems, on which no measurements can be carried out, and closely resemble classical, deterministic evolution of a physical system. The former occur when a measurement is carried out and are indeterministic (or at least probabilistic): when an observable is measured, the state of the system suddenly jumps to an unpredictable state (or "eigenstate") associated with the measured eigenvalue of the observable. Unlike classical Physics, in which the new state can be determined from the prior state of the system, Quantum Theory can only specify the probabilities of moving into any of the observable's eigenstates. In quantum lingo, a measurement causes a "collapse of the wave function", after which the observable assumes a specific value.
Von Neumann pointed out that measurement of a system consists in a process of interactions between the instrument and the system, whereby the states of the instrument become dependent on the states of the system. There is a chain of interactions that leads from the system to the observer's consciousness. For example, a part of the instrument is linked to the system, another part of the instrument is linked to the previous part, and so forth until the interaction reaches the observer's eye, then an interaction occurs between the eye and the brain and finally the chain arrives to the observer's consciousness. Eventually, states of the observer's consciousness are made dependent on states of the system, and the observer "knows" what the value of the observable is.
Somewhere along this process the collapse has occurred, otherwise the end result of the chain would be that the observer's consciousness would exhibit the same probabilistic behavior of the observable: if the observer reads one value on the instrument, it means that the wave function has collapsed somewhere between the system and the observer's consciousness. A continuous process of the first kind must give rise to a discontinuous process of the second kind. Von Neumann concluded that it was not important at what point the process of the first kind occurred.
Consciousness collapses the world.

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