George Berkeley (1685-1753) reacted to the move towards philosophical materialism of Hobbes, Descartes, Locke and Newton:
Some truths there are so near and obvious to the mind that a man need only open his eyes to see them. Such I take this important one to be, viz., that all the choir of heaven and furniture of the earth, in a word all those bodies which compose the mighty frame of the world, have not any subsistence without a mind, that their being (esse) is to be perceived or known; that consequently so long as they are not actually perceived by me, or do not exist in my mind or that of any other created spirit, they must either have no existence at all, or else subsist in the mind of some eternal spirit—it being perfectly unintelligible, and involving all the absurdity of abstraction, to attribute to any single part of them an existence independent of a spirit. To be convinced of which, the reader need only reflect, and try to separate in his own thoughts the being of a sensible thing from its being perceived.
— George Berkeley, A Treatise Concerning the Principles of Human Knowledge, §6.
Berkeley was reacting to the tendency in the light of Newton’s triumph towards extreme materialism. Berkeley never denied that matter existed—only a lunatic could believe that, and Berkeley was eminently sane; Berkeley’s point was that matter couldn’t make up the whole of reality, that reality had an inescapably ideal aspect. Before Galileo this would not have been an issue but the dazzling success of the atomistic natural philosophers called for pushback.
Having encouraged philosophical materialism with Newtonian (classical) physics it is fitting that they should hammer the final nail into the coffin of atomistic physicalism on breaking into the atom, precipitating as it did the breakup of Newtonian physics (as a definitive physical model) and the establishment of quantum-theoretical physics. The results were so shocking that a century later most of our civilization (very much including most scientists, and indeed physicists) still live in a Newtonian world.
You may think that that your mind—your stream of conscious thoughts, ideas, and feelings—influences your actions. You may believe that what you think affects what you do. You could be right. However, the scientific ideas that prevailed from the time of Isaac Newton to the beginning of the twentieth century asserted that your physical actions are completely determined by purely mechanical processes describable in purely mechanical terms. According to that earlier conception of nature, any belief that your conscious choices can make a difference in how you behave is an illusion. You were asserted to be, causally, a mechanical automaton.
We now know that that earlier form of science is fundamentally incorrect.
— Henry Stapp, Mindful Universe: Quantum Mechanics and the Participating Observer, p. vii
The observer (or the mind of the observer) gets irreducibly tangled up in the quantum-mechanics formulation of physical reality (by far the most well tested physical theory we have), Schrödinger devised his famous Cat-in-the-Box thought experiment to illustrate how bizarre this is from the classical physics perspective. In it a thought-cat is locked up in a box with a radioactive source and a ‘diabolical device’ that will kill the cat on detecting a radioactive emission; after there is a 50% likelihood that the radioactive source will have emitted a particle triggering the cat’s demise, a divider is dropped separating the cat from the radioactive source. At this point, according to the quantum mechanics model, the box-and-cat system is in a transposition of many states, 50% of which have the cat dead and the remainder with the cat alive. According to quantum mechanics, this is the state of the system, and to people like Schrödinger and Einstein this was an incomprehensible situation and they concluded that quantum mechanics must be an incomplete theory—that the cat must be really physically dead or alive, but that quantum mechanics is missing the parts to determine the complete state of the situation. Neils Bohr and the Copenhagenists insisted that we have no reason to believe this.
The source of the paradox are the physicalist assumptions combined with an ambivalent attitude to animals (sometimes attributed to Descartes’ placing of the animals in res extensa). To unpick the paradox note that the cat is quite capable of monitoring for itself its own state of health. Only if it is truly believed that the cat is merely an ensemble of quantum particles does the thought experiment become truly puzzling. For those that have found physicalism bizarre—the notion that we are merely a bunch atomic particles pinging around in the void—it is predictable that these kinds of problems must arise. As Berkeley said knowledge of the world is an important and foundational principle that is in no way accounted for by physicalism, and this issue gets forced by quantum mechanics; the wonder is not that quantum mechanics happened but that it took so long, and many still refuse to accept this central fact of physical reality.
The logic of the situation is like the old conundrum of the mythical tree falling over in the woods: does it happen if there is nobody to observe it (Feynman discussed this in The Feynman Lectures, III, 2.6 ‘Philosophical Considerations’). The cat system has by construction been perfectly isolated from the observers so the state of that so-isolated naturally-indeterminate system must be undecided, by construction! Quantum mechanics is being stubbornly consistent in asserting that the state of isolated indeterminate systems remain unknown. I suggest that at the heart of every quantum paradox lies the assumption that there exists a physical reality independent of anyone observing it. Reality isn’t like this.
It is perhaps obvious in retrospect that quantum mechanics should arise once physicists had developed instruments sufficiently sensitive to accurately isolate and measure indeterminate physical systems, allowing the unobserved-tree-falling question to be asked for real, to be confronted with the idealist’s negative answer.