The old dualistic notion of mind and matter, so prominent in Cartesianism, as two radically different kinds of substance, will hardly find defenders to-day. Rejecting this, we are driven to some form of hylopathy, otherwise called monism.

Mind and matter brought down to the essential, to the consciousness and its object, form a natural whole, and the difficulty does not consist in uniting but in separating them.

Reality contains matter and non-matter

The mind-stuff of the world is, of course, something more general than our individual conscious minds.... It is difficult for the matter-of-fact physicist to accept the view that the substratum of everything is of mental character. But no one can deny that mind is the first and most direct thing in our experience, and all else is remote inference.

It is impossible to consider the mechanism of our intellect and the progress of our science without arriving at the conclusion that between intellect and matter there is, in fact, symmetry, concord and agreement. On one hand, matter resolves itself more and more, in the eyes of the scholar, into mathematical relations, and on the other hand, the essential faculties of our intellect function with an absolute precision only when they are applied to geometry.

The concept of substance leads to a materialist aspect of the mind. I speak instead of the spiritual existence of the self without mentioning any 'substance' properties. The great problem is 'how the self controls its brain'. This is dualistic, but not in terms of two substances. Instead it relates to the two worlds of Popper.

This was the true nature of consciousness and the meaning of matter, that ultimately both were one substance without cause or control outside itself.

To elaborate these points in a Western philosophical context, first, the “basic stuff” of the scientific West came to be seen as substances (noun phrases), in which attributes inhere (auxiliary verbs) or which are active (transitive and intransitive verbs). Thus Heraclitus could ask how it was possible to step into the same river twice when all of its substance (water) was different. In attempting to get to the bottom of things, Descartes argued for two substances which share no predicates because they are absolutely distinct: body (that which is extended) and mind (that which thinks). But Cartesian efforts to account for how the two substances could interact were not persuasive to Descartes’ successors. Like Spinoza, Leibniz believed that substances could not, in fact, interact, but while the former concluded there could therefore only be a single substance (with many modes and attributes), the latter instead argued for an indefinitely large number of them. These individual substances, “monads,” could not causally affect each other, but could all dance to the same tune played in a preestablished harmony composed by God.

In this flow, mind and matter are not separate substances. Rather, they are different aspects of one whole and unbroken movement.

Matter and mind have this in common, that certain superficial agitations of matter are expressed in our minds, superficially, in the form of sensations; and on the other hand, the mind, in order to act upon the body, must descend little by little toward matter and become spatialized. It follows that the intelligence, although turned toward external things, can still be exerted on things internal, provided that it does not claim to plunge too deeply.

Intelligence and material process have thus a single origin, which is ultimately the unknown totality of universal flux. In a certain sense, this implies that what have been commonly called mind and matter are abstractions from the universal flux, and that both are to be regarded as different and relatively autonomous orders within the one whole movement...It is thought responding to intelligent perception which is capable of bringing about an overall harmony of fitting between mind and matter.

It is difficult for the matter-of-fact physicist to accept the view that the substratum of everything is of mental character. But no one can deny that mind is the first and most direct thing in our experience, and all else is remote inference — inference either intuitive or deliberate.

The arguments for the two substances - mind and body - have, we believe, entirely lost their validity; they are no longer compatible with ascertained science and clear thinking. One substance with two sets of attributes, two sides (a physical and a mental), a double-faced unity, would appear to comply with all the exigencies of the case.

Without a subject there is no object in the world, without understanding there is nothing to understand, without mind no matter. You think that matter comes first, and then what we call mind. Where is this matter? Where have you ever seen matter? You see oak, fir, slate, and granite, and all sorts of other materies, as the old architects called them, never matter. Matter is the creation of the mind, not the reverse. Our entire world is thought, not wood and stone. We learn to think or reflect upon the thoughts, which the Thinker of the world, invisible, yet everywhere visible, has first thought. What we see, hear, taste, and feel, is all within us, not without. Sugar is not sweet, we are sweet. The sky is not painted blue, we are blue. Nothing is large or small, heavy or light, except as to ourselves. Man is the measure of all things, as an ancient Greek philosopher asserted; and man has inferred, discovered, and named matter. And how did he do it? He called everything, out of which he made anything, matter; materia first meant nothing more than wood used for building, out of which man built his dwelling. Here you have the whole secret of matter. It is building-material, oak, pine, birch, whichever you prefer.