Today it occurred to me that, to me, mental and physical content only really seem different when co-existing. But let's say everything is made of mental content: You're saying the mind and physical world are made of the same substance. If you're a physicalist you agree. What is the characteristic which really makes substance mental rather than physical then? Don't both end up saying that the world can be explained by the substance of the mind and vice versa? What would the actual difference in your ontological commitments be to say that everything is made from mental vs physical substance?


6 Answers 6


Most idealists today postulate an emergence process from the mental to create the physical. Most physicalists today postulate an emergence process to create the mental. These are causal sequence claims, they are different from each other and are testable in principle.

Both imply that phenomena of the primary substrate can exist without the other property. This is again a difference, and is testable in principle.

Neutral monism, the view there is some other substrate that generates both mental and physical, basically holds that neither mental nor physical can exist without the other, which is also a testable claim.

As a dualist, I point to all the unconscious aspects of humans and inert matter to refute idealism and neutral monism, and the examples of ghosts, nde/obe/rv, and reincarnation studies to refute physicalism and neutral monism.

So, these different worldviews have different claims about our world, and are testable. Quine-Duhem applies of course, all worldviews are patchable with enough additional complexity, but testing is possible between them.

  • 1
    "testable in principle" how would one test that?
    – TKoL
    Commented May 2 at 10:35
  • @TKoL I provided examples of test cases of matter that appears not to be conscious and consciousness that appears independent of matter in my answer. Fourth paragraph.
    – Dcleve
    Commented May 2 at 12:00
  • Substance-dualist? How do you address the interaction problem, of how the substances can interact without partaking of each other's qualities?
    – CriglCragl
    Commented May 2 at 12:27
  • @CriglCragl Why do you think that things must “partake of each others qualities” before they can interact? This is a theory that appears not to be true for all sorts of interactions, and is not derivable from logical necessity.
    – Dcleve
    Commented May 2 at 12:39
  • @Dcleve do you have any links for reincarnation studies? That's the only thing I could see in your answer that comes close to the "testable" claim that I could see
    – TKoL
    Commented May 2 at 12:40

This question just plays around with words.

  • As long as you do not specify your term “mental substance” this word has no meaning.

  • In any case the term is misleading. The brain is made up from neurons and other organic substances. The characteristic mental property is the information processing of the brain. It is a category error to call a dynamic process a substance.


There isn't necessarily a difference. However, if you say that all physical phenomena are also mental phenomena and vice versa, that makes you a panpsychist. This is a valid perspective, but not everyone is a panpsychist (though I am).

What is the characteristic which really makes substance mental rather than physical then?

A mental substance would be a substance that grants the mind its peculiar characteristic of subjective experience. Wherever this mental substance is present, there is subjective experience. Wherever this mental substance is absent, there is no subjective experience.

So, note as a consequence of this, that if there is one thing that has subjective experience, and a second thing that lacks subjective experience, at most one (and perhaps zero) of these things could be made of a mental substance.

A physical substance would be a substance that can be measured on scientific instruments. For example: protons, electrons, the electromagnetic field.

Note as a consequence of this, that if there is one thing that can be measured on scientific instruments, and a second thing that is inherently unmeasurable (some claim subjective consciousness cannot be measured), then at most one (and perhaps zero) of these things could be made of a physical substance.

If a physicalist says that there is only physical substance, and no mental substance, he means that there is a substance that can be measured on scientific instruments, and that what grants consciousness is not a matter of which substance the mind is made of, but just a matter of how you arrange of the physical substance. Such a physicalist would also say that plastic bottles (for example) lack consciousness, and thus they cannot be made of a mental substance, and thus the physical substance cannot also be a mental substance.


If we divide the world into nothing but two fundamental entities, mind and matter, then the brain is not a fundamental and is just an instance of matter which houses a particular mind and has the capacity to generate sense qualities that can be experienced by that mind. Matter in general then has the capacity to create phenomenal qualities most likely through some currently unknown psychophysical laws. Most philosophers today believe that these qualities are subjective and mind-dependent but it is possible that qualities are actually physical and it is the perception of them by a mind that is subjective. see sensualism

“sense-data are merely those among the ultimate constituents of the physical world, of which we happen to be immediately aware; they themselves are purely physical, and all that is mental in connection with them is our awareness of them, which is irrelevant to their nature and to their place in physics.” Bertrand Russell


Awareness is known by awareness alone.

  • 1
    From me you have a +1. But I need say that such pithy aphorisms are not liked out here even if they are apt
    – Rushi
    Commented May 2 at 17:39
  • Hello, please be aware that StackExchange has certain standards that may be above the average "discussion forum" (see here. Generally, we expect explanations and/or references for every claim made (even though many people around here forget that these days).
    – Philip Klöcking
    Commented May 2 at 18:26
  • "Awareness is known by awareness alone," is the sole irreducible axiom of reality, to put forth a syllable to the contrary is but self defeating. Awareness is the only constant of all experience, what could be more fundamental to reality than that. What does one need to "reference," beyond that. Reality, as such is an aphorism or a tautology or how ever you want to define it and can only be expressed grammatically as such. If existence exists as something what is that something and if existence exists as all things what knows all things? Awareness alone. Commented May 3 at 22:29


"Mental substance"... maybe tomorrow, i want to design a better wind turbine, or increase the efficiency of a solar panel...

... if everything is physical... i have information I can use to make calculations and predictions. Pressures, densities, frictions, resistances... real world numbers and relationships.

... now lets pretend everything is this "mental substance"... how does one use that? Where are the properties? Characteristics? Predictions about the results of various types of interactions?

Where is the utility of this "mental substance"?? What does it do? What can one do with it? Other than talk about it?

  • What if you want to write a story, or a screenplay. What use is ink and paper, or a keyboard and screen? "Mankind owns four things, that are no good at sea: rudder, anchor, oars, and the fear of going down."
    – Scott Rowe
    Commented May 6 at 10:53
  • @ScottRowe substantialism... a quirky quest. Didja know that until 1774 air was thought to be "a substance"... a single contiguous substance. Then some experimenter proved it to be a gas... molecular. I would have thought such knowledge was older. I was surprised to learn its newness. We really weren't that well informed back in the age of gods. Commented May 6 at 15:14
  • 1
    @ScottRowe p.s. I think I covered story writing under the umbrella "Other than talk about it"... at least in the broader sense. Commented May 6 at 15:18

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