Until not many years ago, the "existence" of a mind or soul would have been passionately denied by most physical scientists. The brilliant successes of mechanistic and, more generally, macroscopic physics and of chemistry overshadowed the obvious fact that thoughts, desires, and emotions are not made of matter, and it was nearly universally accepted among physical scientists that there is nothing besides matter.

This is Laplace's superintelligent "demon"

The epitome of this belief was the conviction that, if we knew the positions and velocities of all atoms at one instant of time, we could compute the fate of the universe for all future. Even today, there are adherents to this view though fewer among the physicists than — ironically enough — among biochemists.

There are several reasons for the return, on the part of most physical scientists, to the spirit of Descartes's "Cogito ergo sum," which recognizes the thought, that is, the mind, as primary. First, the brilliant successes of mechanics not only faded into the past; they were also recognised as partial successes, relating to a narrow range of phenomena, all in the macroscopic domain.When the province of physical theory was extended to encompass microscopic phenomena, through the creation of quantum mechanics, the concept of consciousness came to the fore again: it was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness.

Here Wigner is describing the "Schnitt" or "cut" of Werner Heisenberg and John von Neumann.
John Bell called it a "shifty split"

All that quantum mechanics purports to provide are probability connections between subsequent impressions (also called "apperceptions") of the consciousness, and even though the dividing line between the observer, whose consciousness is being affected, and the observed physical object can be shifted towards the one or the other to a considerable degree, it cannot be eliminated. It may be premature to believe that the present philosophy of quantum mechanics will remain a permanent feature of future physical theories; it will remain remarkable, in whatever way our future concepts may develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality.

("Remarks on the Mind-Body Question," in Symmetries and Reflections, p.171)

Wigner complicated the problem of the "Schnitt" of von Neumann (or the "shifty split" of John Bell) that forms the dividing line between the quantum world and the classical measurement apparatus. Wigner moved it farther into the conscious mind of the observer.

Wigner is often said to have extended the problem of Schrödinger's Cat, by adding a second observer inside the laboratory who is commonly known as Wigner's Friend. Popular treatments of Wigner's Friend usually describe him as observing a superposition of live and dead cat. Actually, Wigner's example was a photon and whether its wave function collapsed to cause a flash visible to his friend or not. Wigner's goal was to show that only consciousness can collapse a wave function.

Let's use the cat example, because it is more vivid. You can see Wigner's original argument on the Wigner's Friend page..

The physicist friend inside the lab opens the box and observes either a live or dead cat. But Wigner is outside the lab and does not know the outcome. Wigner says this seems to leave the world in a superposition of states - "dead cat/sad friend" and "live cat/happy friend."

Wigner says that any inanimate material measuring device is left in a superposition of states. This would include his friend and himself, but for human consciousness. He resolves his paradox by saying that consciousness collapses the wave function, both his friend's inside the laboratory and his own.

The information interpretation of quantum mechanics helps to resolve this paradox as follows,

• If the physicist friend inside the lab clearly looks inside the box and is seen to record the result in a notebook, we can safely conclude that the superposition of states has "collapsed" (been projected) into either the dead cat or live cat state.

• In the case of the Geiger counter detecting the nuclear decay, it has released an irreversible avalanche of electrons, which makes a macroscopic recording of the event and provides the energy to break the vial of cyanide.

  

• New information has been created in the universe. Entropy has been radiated away, so the change is irreversible.

• We can assume that an observation has been made, recorded as a measurement, and, to satisfy Wigner, von Neumann, Wheeler, Bell. and others, the measurement has entered the mind of the "conscious observer," though our information interpretation does not require this step.The wave-function collapse occurs with the creation of new information, without the need for observations or measurements.

  John Bell quipped whether the conscious observer needed to have a Ph.D. to collapse a wave function. Here is a diagram that Bell drew of the shifty split, with an annotation where the information interpretation of quantum mechanics says that the collapse occurs (no observers necessary).

  

• Since Wigner does not know the actual outcome, he only knows the possibilities and can estimate ordinary probabilites, for example, that there is a 75% chance the cat is dead and 25% probability the cat is alive.

• But here is the resolution of Wigner's paradox. These probabilities are no longer about superposed quantum states interfering with one another. They are no longer quantum probabilities. The cat is either dead or alive! The chances are no longer ontological. They are epistemic, just human ignorance.

• So Wigner is wrong to conclude that the cat remains in a quantum superposition of live and dead cat states.

• Nor was the cat ever in such a superposition! After all, the cat is animate - and conscious. It was our calculations of nuclear decay that used the quantum superposition as our best estimates. And it was important to include the possible interference effects while the wave functions (for the decaying nucleus) were still coherent. Once we get information about the nuclear decay, the wave functions decohered and we must switch to "classical" probabilities.

Wigner was rare among physicists in mentioning conservation laws in his discussion of the Einstein-Podolsky-Rosen experiment.