Figuring in the epistemological dualism of subject vs. object was the parallel but ontological dualism of spirit vs. matter, or mental vs. material. This dualistic problem revolved around trying to decide of what basic "stuff" the universe was composed: was it all nothing but material atoms, arranged in such a way that consciousness was just an illusion, being in reality reducible to the interplay of physical particles so that "mind" is really just a conglomeration of matter? But what of the argument that all sensations of "matter" exist nowhere but in somebody's mind—doesn't that demonstrate that matter is really nothing but an idea? Ever since Plato separated ideas from experience, the argument as to which is "really" real has continued, with no side clearly winning.
Is consciousness really matter, or is matter really consciousness? The idealists, or mentalists, just could not stomach the thought that consciousness was not much more than a fancy lump of clay, differing not at heart from rocks, tables, and dirt; thus, they were always on hand with the question, "But where does the impression of matter have its existence?" The answer, of course, is that material impressions exist only in consciousness, and so the conclusion is obvious: all matter is but a mental idea. This, however, was too much for the materialists, who would reply, "Well, then, where does consciousness come from?” The answer here being, "From nothing but physical processes in the human brain," and so the opposite conclusion is equally obvious: all ideas are just material. Emotions were high, for both sides of the argument could be put with equal persuasion, and so the final decision usually rested upon individual inclination, as is shown in the following story told by Eddington:
When Dr. Johnson felt himself getting tied up in argument over “Bishop Berkeley's ingenious sophistry to prove the non-existence of matter, and that everything in the universe is merely ideal," he answered, "striking his foot with mighty force against a large stone, till he rebounded from it, — 'I refute it thus.' " Just what that action assured him of is not very obvious; but apparently he found it comforting. And to-day the matter-of-fact scientist feels the same impulse to recoil from these flights of thought back to something kick-able, although he ought to be aware by this time that what Rutherford has left us of the large stone is scarcely worth kicking.
As this story hints, the old science had allied itself with the materialists, for lumps of matter could be "kicked," that is, measured and verified, whereas no scientist had come up with any sort of instrument capable of recording spirituality. The new quantum physicists didn't argue with this—they certainly couldn't find any spiritual stuff either—but, and here is the point, neither could they find any material stuff. As one physicist put it:
Our conception of substance is only vivid so long as we do not face it. It begins to fade when we analyze it... the solid substance of things is another illusion. We have chased the solid substance from the continuous liquid to the atom, from the atom to the electron, and there we have lost it.
And Bertrand Russell summed it up succinctly—"The world may be called physical or mental or both or neither as we please; in fact the words serve no purpose." In short, quantum physics had taken another dualism, that of mental vs. material, to the annihilating edge, and there it had vanished.
The Spectrum of Consciousness, Ken Wilber, Chapter 2