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In 1961, Eugene Wigner introduced the so-called "Wigner's friend" thought experiment as a plausible demonstration of the effect of the consciousness on the physical world, or more specifically, the Von Neumann-Wigner interpretation of wavefunction collapse in quantum mechanics. In the paper he elaborates this argument of his in detail, but at the end of it, offers another much more cryptic argument that he nonetheless claims to be convincing:

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. It is true that under the usual conditions of experimental physics or biology, the influence of any consciousness is certainly very small. "We do not need the assumption that there is such an effect." It is good to recall, however, that the same may be said of the relation of light to mechanical objects. Mechanical objects influence light—otherwise we could not see them—but experiments to demonstrate the effect of light on the motion of mechanical bodies are difficult. It is unlikely that the effect would have been detected had theoretical considerations not suggested its existence, and its manifestation in the phenomenon of light pressure.

What does he mean by this?

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  • I don't see how that's an argument for dualism. It's a rejection of one kind of dualism, namely parallelism. It doesn't seem to have occurred to Wigner that the influence of consciousness on physical objects is anything but small in everyday life.
    – benrg
    Oct 1, 2023 at 0:54
  • @benrg Your language is probably more accurate for what he means to say, and I'll update the Q. I doubt it got past Wigner that the influence is small (he acknowledges no experiments to date have captured such an influence), but it seems to him its existence is both necessary, and of great significance.
    – Feryll
    Oct 1, 2023 at 2:02
  • He means that we do not know of any phenomenon where the law of action and reaction fails. If X "influences" Y then there is a reciprocal "influence" of Y on X. Since the world (X) clearly "influences" consciousness (Y) there should be a reciprocal "influence" of that consciousness on the world. He then analogizes it to the "influence" of light on "mechanical bodies", which is, typically, imperceptible, but can be detected as light pressure when one is looking for it.
    – Conifold
    Oct 1, 2023 at 5:19

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Clearly consciousness has an influence on the physical world- how else am I able to type this answer? The (IMHO utterly unconvincing) point he is making is that there are subtle effects- such as light pressure- which are readily overlooked but are nonetheless implied by Newton's third law, so by analogy there may be overlooked effects relating to the interaction of consciousness with matter. What the Wigner's friend thought experiment demonstrates is not that consciousness has a special role in relation to the 'collapse' of the wave-function, but that the Von Neumann interpretation of QM cannot sensibly be applied to everyday objects. The glaring approximation in the conventional interpretation of QM is the asymmetrical treatment of the object being observed- which is assumed to be a quantum object- and the measuring device, which is assumed not to be. In reality, of course, when an electron, say, interacts with a measuring device, such as a photographic plate, it is interacting not with a classical object but with the photons, ions and electrons that comprise the plate, all of which are quantum objects themselves. The complexity of those interactions are entirely neglected in classical QM, which treats the observed electron as having a wave function and the plate as an external influence that magically causes the electron wave function to change, rather than considering the fact that the electron and all of the individual particles comprising the detector have an associated multi-particle wave-function which evolves.

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