Editing Wigner's friend (section)

===The observer observed===
Wigner presents two arguments for the thesis that the mind influences the body, i.e., that a human body can "deviate from the laws of physics" as deduced from experimenting upon inanimate objects. The argument that he personally finds less persuasive is the one that has become known as "Wigner's friend". In this thought experiment, Wigner posits that his friend is in a laboratory, and Wigner lets the friend perform a quantum measurement on a physical system (this could be a [[Spin (physics)|spin system]]). This system is assumed to be in a [[Quantum superposition|superposition]] of two distinct states, say, state&nbsp;0 and state&nbsp;1 (or <math>|0\rangle</math> and <math>|1\rangle</math> in [[Dirac notation]]). When Wigner's friend measures the system in the {0,1}-[[Basis (linear algebra)|basis]], according to quantum mechanics, they will get one of the two possible outcomes (0 or 1) and the system will [[Wave function collapse|collapse]] into the corresponding state.

Now Wigner himself models the scenario from outside the laboratory, knowing that inside, his friend will at some point perform the 0/1-measurement on the physical system. According to the linearity of the quantum mechanical equations, Wigner will assign a superposition state to the whole laboratory (i.e. the joint system of the physical system together with the friend): The superposition state of the lab is then a linear combination of "system is in state 0 — friend has measured 0" and "system is in state 1 — friend has measured 1".

Let Wigner now ask his friend for the result of the measurement. Whichever answer the friend gives (0 or 1), Wigner would then assign the state "system is in state 0 — friend has measured 0" or "system is in state 1 — friend has measured 1" to the laboratory. Therefore, it is only at the time when he learns about his friend's result that the superposition state of the laboratory collapses.

However, unless Wigner is considered in a "privileged position as ultimate observer", the friend's point of view must be regarded as equally valid, and this is where an apparent [[paradox]] comes into play: From the point of view of the friend, the measurement result was determined long before Wigner had asked about it, and the state of the physical system has already collapsed. When exactly did the collapse occur? Was it when the friend had finished their measurement, or when the information of its result entered Wigner's [[consciousness]]? As Wigner says, he could ask his friend, "What did you feel about the [measurement result] before I asked you?" The question of what result the friend has seen is surely "already decided in his mind", Wigner writes, which implies that the friend–system joint state must already be one of the collapsed options, not a superposition of them. Wigner concludes that the linear time evolution of quantum states according to the [[Schrödinger equation]] cannot apply when the physical entity involved is a conscious being.

Wigner presents his second argument, which he finds more persuasive, much more briefly:
: The second argument to support the existence of an influence of the consciousness on the physical world is based on the observation that we do not know of any phenomenon in which one subject is influenced by another without exerting an influence thereupon. This appears convincing to this writer.