The founders of quantum mechanics debated the role of the observer, and of them,
Wolfgang Pauli and
Werner Heisenberg believed that it was the observer that produced
collapse. This point of view, which was never fully endorsed by
Niels Bohr, was denounced as mystical and anti-scientific by
Albert Einstein. Pauli accepted the term, and described quantum mechanics as
lucid mysticism.
[1]
Heisenberg and Bohr always described quantum mechanics in
logical positivist terms. Bohr also took an active interest in the philosophical implications of quantum theories such as his
complementarity, for example.
[2] He believed quantum theory offers a complete description of nature, albeit one that is simply ill suited for everyday experiences - which are better described by classical mechanics and probability. Bohr never specified a demarcation line above which objects cease to be quantum and become classical. He believed that it was not a question of physics, but one of philosophy.
Eugene Wigner reformulated the "
Schrödinger's cat"
thought experiment as "
Wigner's friend" and proposed that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function, independent of any realist interpretation. Commonly known as "
consciousness causes collapse", this
interpretation of quantum mechanics states that
observation by a
conscious observer is what makes the
wave function collapse.