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?