Max Planck, Schrodinger, Bohr, and other founders of quantum mechanics have all expressed an idealist or subjective idealist position: in other words, they believed reality is created by or dependent on the mind. I see why quantum mechanics lends itself to scientific antirealism, but if you were to accept it as just instrumental I don't see why you would then be committed to idealism. So what was their reasoning?

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    in other words, they believed reality is created by or dependent on the mind - would you mind citing sources for this?
    – TKoL
    Commented Jan 25 at 17:55
  • "As a man who has devoted his whole life to the most clearheaded science, to the study of matter, I can tell you as a result of my research about the atoms this much: there is no matter as such! All matter originates and exists only by virtue of a force which brings the particles of an atom to vibration and holds this most minute solar system of the atom together… We must assume behind this force the existence of a conscious and intelligent mind. This mind is the matrix of all matter." - Max Planck I've seen this quoted quite a bit but I'm not entirely sure of the source.
    – edelex
    Commented Jan 25 at 18:12
  • where have you been reading it, goodreads? can easily be googled @edelex
    – user71226
    Commented Jan 25 at 18:15
  • phenomenalism was important to the development of QM (i have been told). why it was, i don't know. i doubt it was the science behind it, anymore than falsificationism is why we have cern.
    – user71226
    Commented Jan 25 at 18:18
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    You don't "found" a scientific theory, you propose and verify it based on empirical observation. We did that with QM, and that's why we accept it. That's perfectly consistent with scientific realism and materialism (regardless of the views of those who proposed it). If you can't measure something without affecting its state, that seems to suggest that the method of measurement is destructive, more than it suggests that nothing really exists (the latter carries a heavy epistemic burden, in that now you have to make sense of why everything else seems to be measurable without affecting state).
    – NotThatGuy
    Commented Jan 26 at 5:14

2 Answers 2


I suppose one of the reasons for their idealism1 is a particular interpretation of quantum mechanics, which also happens to be the commonly accepted one. Basically, it suggests that the state of the Universe is undefined until someone looks. Each particle appears to be everywhere, filling up the entire Universe2 -- until, again, someone tries to actually observe it. Once they do, however, they always find the particle at some specific location, with the probability determined by the so-called wave function.3

The particle's transition from being everywhere to being found at a specific point in space is referred to as wave function collapse. And again, the naive interpretation is that the collapse is triggered by the act of observation, thus suggesting that the sentient(?) observer creates (or shapes, or forms) the reality.

Some prominent physicists -- most notably Albert Einstein -- never bought into this idea. It should be also noted that no one has ever seen a particle in its undefined state. Indeed, eventually we came up with other interpretations of QM that do not require an observer for the Universe to take its shape (most notably, the many worlds interpretation, which, ironically, does not require many worlds -- the multiverse -- to actually exist either).

However, in the early days, the naive interpretation was only one of the two available (the other being "shut up and calculate"). Finally, there were other serious, if personal, reasons why many found (and still find) this interpretation appealing.

1 Alleged -- and I think Bohr was less guilty of it. His position was closer to "shut up and calculate".

2 This crazy idea comes from experiments, in particular the so-called double-slit experiment. In it, a single particle has to pass through a solid plate with two slits in it. One would think that it can only pass through one slit or the other. In practice, however, the particle acts as if it knew whether only one or both slits are open -- as if it went through both slits on its way through the plate.* In fact, it appears as if the particle took every possible path -- also going around the Moon, or around the most distant galaxy, etc. -- the speed of light limit be damned.

* And this we know because the probability of the particle being observed at specific location changes depending on whether one or both slits are open.

3 Every particle and every chunk of matter has its wave function. The universal wave function determines the probable position of every particle in the Universe.

  • Most physicists today do not distinguish "observer" from "interaction." Interactions collapse the waveform in Heisenberg. Many Worlds does not actually change this -- the lack of location pre-interaction, and the interaction event causing a major phase change in the world -- are both part of MWI as well. All MWI does instead is keep the other locations too, in absurdly plentiful and purely invented and unobservable "alternate worlds".
    – Dcleve
    Commented Jan 26 at 15:43
  • +1 but be careful not to muddle "defining space" with "observing space" and "describing space". You> "Basically, it suggests that the state of the Universe is undefined until someone looks." Definition is primarily a semantic act; description is primarily syntactical; and observation is primarily conceptual. We define space and matter with wave functions, observe it with experimentation to see if our empirical experience aligns with our definition, and then describe the results to tentatively confirm or possibly falsify our definition.
    – J D
    Commented Jan 26 at 18:13
  • @Dcleve -- thank you, that's a good point. Indeed, if we replace observation with interaction, we end up with MWI minus many words. Note, however, that since we cannot interact with other words, their actual existence/creation cannot be a requirement, not even in MWI. In other words, the many worlds story is a parable -- a story we tell not to state that it actually happened, but to illustrate a certain point that we are trying to make. Commented Jan 26 at 19:07
  • @JD -- I agree that "defined" is not an ideal word. But then again, that's the thing about human language -- it's somewhat imprecise, and think it's by design. Instead of looking for the ideal word, we should rely on a dialog for clarifying the meaning of what was said through iterations of follow up questions and answers. That's why, I think, some old philosophical text were structured that way. Commented Jan 27 at 19:12
  • @Dcleve "Interactions collapse the waveform in Heisenberg." -- actually, that does not make sense because it does not explain how two particles can interact before their wavefunctions collapse? Or, as another way to ask this question -- what collapses the universal wavefunction (the wavefunction of the entire Universe)? Commented Apr 29 at 2:21

The development of modern physics shattered the old dichotomy between perceptual idealism, and materialist realism.

The classical refutation of idealism was by Samuel Johnson kicking a stone. This was not decisive, https://www.essentiafoundation.org/idealist-stones-hurt-too/reading/#:~:text=Johnson%20is%20implying%20that%20it,stones%20does%20not%20refute%20idealism. but it does illustrate the options that were under debate.

Under modern physics, consisting of QM and relativity, BOTH our perception and our "real" world of the matter around us, are mistaken. This view is called scientific realism, and it holds that what is "real" are the fundamental objects and equations of physics -- and that "rock" is just a transitory artifact, with ill-defined boundaries, and "really" consisting of mostly empty space, that the principles of Electro Magnetism prevented Johnson's foot from taking advantage of and passing through the "rock". Note under scientific realism, "matter" is no longer fundamental -- it too is a transient and ill-defined manifestation of the more fundamental principles of energy, entropy, momentum, probability fields, etc. Note all of these "real" things in scientific realism fall under the general class of "abstractions". Scientific realism basically holds that matter reduces to math, and is a form of platonism. "Shut up and calculate" is a conclusion of a belief in the fundamental "reality" of the math behind physics.

Counter proposals to scientific realism include instrumentalism, which holds that energy, momentum, entropy, and probability fields are just useful ciphers in our minds to deal with the "real world". This has been seized upon by materialists to challenge "scientific realism", but this tactic is poorly thought thru. What is "real" in instrumentalism are the experiences we make our inferences from, and the mind that holds them as useful instrumental models. Matter, and the world around us, is as much an "instrumental speculation" as the complexities of modern physics are. Instrumentalism is actually a classical phenomenological idealism.

One of the other objections to scientific realism was the criticality of an observer in the tests that established QM. QM today is generally treated as having wave function collapse when there are "interactions" rather than "observations" (which require an observer, putting US directly as agents in the "reality of the world). But this is incorrect. Einstein forced this difference into physics attention with one of his later papers. https://plato.stanford.edu/entries/qt-epr/

EPR led to Bell demonstrating his inequality. Per Bell's Inequality, QM cannot be both "local" and "real", where local is defined as satisfying the light cone interaction boundaries of relativity, and "real" is defined as a mind-independent reality. Bell's Inequality has been tested, and confirmed. This confirmation is widely interpreted as a refutation of the "locality" of light cone boundaries of relativity, but it could alternatively be treated as a refutation of "realism".

Another poster proposed MWI as another alternative, but this is a misunderstanding of MWI. MWI is fully subject to Bells Inequality, and does not rescue classical realism from the challenges it faces due to modern physics. Note also that MWI is just another variant on the platonism of scientific realism, where "math is what is real" is taken to an even greater extreme, and an infinity of universes are proposed because the math is cleaner if they split rather than collapsing.

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