5.2.2 Metaphysical Monism


By the end of this section you will discover:

  • The two possible options for metaphysical monism.
  • Why the principle of Ockham’s Razor seems to support monism over dualism.
  • Strengths and weaknesses of the materialist position.
  • Strengths and weaknesses of the idealist position.

The alternative to the position that reality is made of two kinds of substances we call metaphysical monism.  Dualistic views, according to 20th-century philosopher Gilbert Ryle (1900-1976), make the mind into a mere “ghost in the machine,” an idle wheel, turning nothing. The interaction problem, in fact, probably helps to explain why philosophers haven’t often been drawn to theories according to which there are three, four, or even more basic kinds of substance.

The only surefire way to avoid the interaction problem is to claim that all of reality is fundamentally one kind of thing. Approaches like this are versions of metaphysical monism. We saw examples of this kind of monism earlier with Thales (water) and Anaximander (air). But the most influential versions of monism in the past millennium have been materialism and idealism. Materialism claims that all of reality is fundamentally physical or material. Idealism claims that all of reality is mental, i.e., all that exists are minds and their ideas. Those approaches will be the topics of our next two sections.

Metaphysical Materialism

Materialism itself has had philosophical adherents since ancient times. Democritus, who was mentioned earlier, claimed that all that exists is atoms (i.e., physical entities) and void. We see similar thoughts expressed in this poetic passage from ancient Roman philosopher Lucretius (c. 99 – c. 55 BCE):

But, now again to weave the tale begun,

All nature, then, as self-sustained, consists

Of twain of things: of bodies and of void

In which they’re set, and where they’re moved around.

For common instinct of our race declares

That body of itself exists unless

This primal faith, deep-founded, fail us not,

Naught will there be whereunto to appeal

On things occult when seeking otherwise to prove

By reasonings of mind. Again, without

That place and room, which we do call the inane,

Nowhere could bodies then be set, nor go

Hither or thither at all—as shown before.

Besides, there’s naught of which thou canst declare

It lives disjoined from body, shut from void—

A kind of third in nature. For whatever

Exists must be a somewhat; and the same,

If tangible, however, fight and slight,

Will yet increase the count of body’s sum,

With its own augmentation big or small…

(Lucretius, On the Nature of Things, Project Gutenberg)

Taking it to the Streets…

Ask several friends what the universe is “made of.”

What kinds of answers do you get?

How many of them included both physical realities and non-physical?

If they forgot to include the latter, ask them first whether things like thoughts are real, and second if they believe these things are physical.  Ask the same for the soul, for God, for Mathematics.

How do they explain the existence of such things if all that “is” is physical?

Though materialism could be found sporadically in the ancient and medieval worlds, it became much more popular during and after the scientific revolution, as contemporary philosopher Heather Salazar, in her chapter “Materialism and Behaviorism,” explains:

The scientific revolution began in the mid-sixteenth century and the progress of science throughout the nineteenth century made science a proven method of quick advancement for knowledge. Some philosophers, such as David Hume…, argued that people should “reject every system … however subtle or ingenious, which is not founded on fact and observation.” … Descartes … (who argued for substance dualism) and John Locke … had … theories that tried to forward philosophical views within science, then called mechanical philosophy, which sought to find explanations that were subject to physical laws. Whereas Descartes was a rationalist, relying on principle, Locke was an empiricist and relied on experience (constituting evidence). Both Descartes and Locke had to prove that their theories were consistent with God and the religion of the time (which in Europe was [predominantly] Christianity); however, later theorists either left God completely out of the picture or tried to show from a theoretical basis that there still was a place for God in science.

Some of the important foundations of science, such as the closure principle and the primacy of the empirical over the theoretical, were prominent in philosophy, as well. In the sciences, experiments and theories rely centrally on the closure principle, which states that material objects have causes and effects that are locatable in the physical world. Without this principle, there would be no reason to do scientific research. Instead of claiming that the cause of a disease is a virus, we could just as easily claim that it is caused by God’s wrath or a demonic force. This slowly caused people to rethink their ideas of the existence of God. If God was no longer needed to explain the things that we experience in the world—if science could do it completely without the use of God—then why do we need to believe in the existence of God?

An empiricist will readily point out that you cannot see God, nor can see your mind. You may be able to see someone else’s brain if you witness a surgeon operating on someone, but you cannot see anyone’s mind, including your own. And according to the principle of closure, something that is immaterial cannot affect something that is material, so the brain or other physical things are more properly the cause of our actions, not some mystical immaterial substance of the mind.

The principle of Ockham’s Razor—named after William of Ockham (1285-1347), a philosopher from the middle ages—states that when something of a different kind (in this case, immaterial things) is not needed to explain something else (material things), then it can be eliminated. Favored in the sciences, Ockham’s Razor is an explanatory principle of parsimony, and it gave philosophers a justification to remove God and other items that could not be seen (like minds) from their ontological status as real (separate) objects.

Ponder if you will….

an image of a razor blade with Ockham's name on it

Which is the simpler explanation:

You see a sudden flare of bright light in the sky. Is it most likely a UFO or lightning.?

You see birds flying over your house. Is it most likely they are just birds or has the government created surveillance drones in the shape of birds to spy on us?

You have a thought. Is it more likely that your brain fired a specific set of neurons, or that a non-physical mind gave you an idea?

How would Ockham choose?

Instead, talk of minds and mental events, such as thoughts and feelings, are simply shorthand expressions for processes in the body and world that science helps people to understand. It is therefore reasonable, they thought, that either our minds really are just bodies or else minds do not exist. Ockham’s Razor became the battle cry of the new materialist brand of philosophers, scientists, and psychologists in the modern era and even today. (Salazar, “Materialism and Behaviorism,” in Salazar, et.al.)

We will discuss materialist theories of the mind in much more detail in the Philosophy of Mind section below. Materialists, as we will see, have a lot of work to do to explain how the mind and its contents could be physical. It turns out, as we’ll see later, that both consciousness (awareness) and intentionality (volition) have proven difficult to explain in purely material terms.

But before getting there, and before moving on to discuss idealism, it is important to note that materialism has a hard time explaining several different kinds of entities, not only minds. There are at least two kinds of things that have proven difficult to accommodate in a materialist framework: values and what philosophers call abstract objects (e.g., things like numbers, concepts, formulae, and sets).

The problem with the second category of objects is described nicely by contemporary philosopher Daniel Stoljar in the Stanford Encyclopedia of Philosophy:

… the problem of abstracta…concerns the status within [materialism] of abstract objects, i.e., entities apparently not located in space and time, such as numbers, properties, relations, or propositions.

To see the problem, suppose that abstract objects, if they exist, exist necessarily, i.e., in all possible worlds. If [materialism] is true, then the facts about such objects must either be physical facts or else bear a particular relation (grounding, realization) to the physical. But on the face of it, that is not so.

Can one really say that 5+7=12, for example, is realized in, or holds in virtue of, some arrangement of atoms and void? Or can one say that it itself is a physical fact or a fundamental physical fact? If not, physicalism is false: the property of being such that 5+7=12 [functions] the actual world but is neither identical to, nor grounded in, or realized by, any physical property.  (SEP, “Physicalism,” Section 5.3)

The problem of values is similar. For example, brutally beating someone, just to make one of your deranged friends laugh, is wrong. Different moral theories will explain that wrongness in different ways: Kantians will say that it’s wrong because you treated the person merely as a means; utilitarians will say that it’s wrong because not beating this person would have brought more net pleasure into being; virtue ethicists will say that it’s wrong because, in so acting, you manifested serious character flaws involving cruelty, etc. (See our Ethics chapter)

But the fact that your action was wrong does not seem like just another physical fact of the universe, like the facts about where you did it, how fast your fists moved, etc… Facts about what’s good or bad, what’s right or wrong, and what we ought or ought not to do, seem to be utterly unlike biological. chemical, or physical facts.

Moral facts – if such there be – make demands on us: if you acknowledge that pain is bad, then you acknowledge that you are obligated not to cause it. However, we can acknowledge ordinary physical facts – e.g., how tall a tree is, how quickly traffic is moving on the main road, how much it costs to buy a six-pack of beer, etc. – without acknowledging any obligation whatsoever. Facts about values are normative, as philosophers say, whereas ordinary physical facts are not. So here we have another potential problem for materialism.

Metaphysical Idealism

The other primary version of metaphysical monism (for at least the last several hundred years) turns materialism on its head, so to speak. According to idealism, all of reality is mental or spiritual in nature, not physical at all; all that exists are minds and their ideas. “Body” as some kind of external, physical “thing” does not exist. However, the body as perceived by the mind may indeed be real, just not physical. What we take to be examples of body (e.g., our arms and legs, cows, bricks, water, etc.) are, according to the Idealist, ideas.

While dualism and materialism both have quite a bit of everyday, commonsense appeal, idealism requires additional explanation and argument. How is this pencil I’m holding in my hand an “idea”? If it is just an idea, does that mean that it stops existing when nobody is perceiving it or thinking about it? If you and I are just minds, and we don’t inhabit a shared physical space, how do we interact with one another?

Idealism – like materialism – was known in the ancient world. But it got a new impetus in the 17th century, as scientists began learning more and more about how our perceptual systems actually worked, and we started to understand that many of the properties that seemed to inhere in external objects (e.g., colors, odors, textures—what Locke called the “Secondary Qualities” of percepts—see our chapter on Epistemology) might not be so mind-independent after all. Hume furthered this understanding. To paraphrase one of his most famous claims about values, it began to seem, in a number of cases, as though our perceptual faculties did not present to us reality as it is in itself, but they played a large role in “gilding or staining all natural objects with the colors, borrowed from internal sentiment” (Hume, An Enquiry Concerning the Principles of Morals, Appendix I.)

George Berkeley (1685-1753) was no doubt the most influential idealist philosopher of the early modern era. In the following selection, he lays out the case(s) for idealism and responds to several objections.

Excerpts from Berkeley’s Treatise Concerning the Principles of Human Knowledge

In what follows, consider:

What does Berkeley say about those who believe that the objects of perception are somehow distinct or separate from the mind?

It is indeed an opinion strangely prevailing amongst men, that houses, mountains, rivers, and in a word all sensible objects, have an existence, natural or real, distinct from their being perceived by the understanding. But …whoever shall find in his heart to call [this] in question may, if I mistake not, perceive it to involve a manifest contradiction. For, what are the fore-mentioned objects but the things we perceive by sense? and what do we perceive besides our own ideas or sensations? and is it not plainly repugnant that any one of these, or any combination of them, should exist unperceived?

How does he explain why so many make this error?

If we thoroughly examine this tenet it will, perhaps, be found at bottom to depend on the doctrine of abstract ideas. For, can there be a nicer strain of abstraction than to distinguish the existence of sensible objects from their being perceived, so as to conceive them existing unperceived? Light and colors, heat and cold, extension and figures—in a word the things we see and feel—what are they but so many sensations, notions, ideas, or impressions on the sense? and is it possible to separate, even in thought, any of these from perception? For my part, I might as easily divide a thing from itself… But my conceiving or imagining power does not extend beyond the possibility of real existence or perception. Hence, as it is impossible for me to see or feel anything without an actual sensation of that thing, so is it impossible for me to conceive in my thoughts any sensible thing or object distinct from the sensation or perception of it.

What, according to Berkeley, does it mean to say that something exists?

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 (est percipi) 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.

How does Berkeley refute the objection that ideas are “like” the things outside the mind?

“But” say you, “though the ideas themselves do not exist without the mind, yet there may be things like them, whereof they are copies or resemblances, which things exist without the mind in an unthinking substance.” I answer, an idea can be like nothing but an idea; a color or figure can be like nothing but another color or figure. If we look but never so little into our thoughts, we shall find it impossible for us to conceive a likeness except only between our ideas. Again, I ask whether those supposed originals or external things, of which our ideas are the pictures or representations, be themselves perceivable or no? If they are, then they are ideas and we have gained our point; but if you say they are not, I appeal to any one whether it be sensible to assert a color is like something which is invisible; hard or soft, like something which is intangible; and so of the rest.

How does he refute the notion that primary or secondary qualities tell us anything about the external world (i.e., representative realism)?

What is the problem with the concept of “matter”?

Some there are who make a distinction betwixt primary and secondary qualities. [Ed: Here Berkeley is referring to John Locke. For more information about Locke, see the Empiricists section of our Epistemology chapter.] By the former they mean extension, figure, motion, rest, solidity or impenetrability, and number; by the latter they denote all other sensible qualities, as colors, sounds, tastes, and so forth. The ideas we have of these they acknowledge not to be the resemblances of anything existing without the mind, or unperceived, but they will have our ideas of the primary qualities to be patterns or images of things which exist without the mind, in an unthinking substance which they call matter. By matter, therefore, we are to understand an inert, senseless substance, in which extension, figure, and motion do actually subsist. But it is evident from what we have already shown that extension, figure, and motion are only ideas existing in the mind, and that an idea can be like nothing but another idea, and … hence, it is plain that the very notion of what is called matter or corporeal substance involves a contradiction in it.

Even if material things do exist, how could we ever know them?

But, though it were possible that solid, figured, movable substances may exist without the mind, corresponding to the ideas we have of bodies, yet how is it possible for us to know this? Either we must know it by sense or by reason. As for our senses, by them we have the knowledge only of our sensations, ideas, or those things that are immediately perceived by sense, call them what you will: but they do not inform us that things exist without the mind…. This the materialists themselves acknowledge. It remains therefore that if we have any knowledge at all of external things, it must be by reason, inferring their existence from what is immediately perceived by sense. But what reason can induce us to believe the existence of bodies without the mind, from what we perceive…. I say it…is possible that we might be affected with all the ideas we have now, though there were no bodies existing without resembling them!

How does Berkeley suggest that Materialism is thought by some to be an explanation of the cause of our sensations? What does he see as the main fault of this supposition?

But, though we might possibly have all our sensations without them, yet perhaps it may be thought easier to conceive and explain the manner of their production, by supposing external bodies in their likeness rather than otherwise; and so it might be at least probable there are such things as bodies that excite their ideas in our minds. But neither can this be said; for, though we give the materialists their external bodies, they by their own confession are never the nearer knowing how our ideas are produced; since they own themselves unable to comprehend in what manner body can act on spirit, or how it is possible it should imprint any idea in the mind. Hence it is evident the production of ideas or sensations in our minds can be no reason why we should suppose Matter or corporeal substances, since that is acknowledged to remain equally inexplicable with or without this supposition.

The very idea of matter seems to involve a contradiction, insofar as it is supposed to be “something” that we can perceive, yet we can never perceive anything which might exist outside of our (mental) perception! With these arguments for immaterialism—the philosophy that there is no “matter” existing outside the mind—Berkeley lays the groundwork for his idealism. However, cannot our minds be deceived? How does Berkeley suggest that our mental contents are not mere illusions?

How does Berkeley distinguish between the mental contents of the senses and those of more abstract ideas?

The ideas imprinted on the Senses by the Author of nature are called “real things”; and those excited in the imagination being less regular, vivid, and constant, are more properly termed ideas, or images of things, which they copy and represent. But then our sensations, be they never so vivid and distinct, are nevertheless ideas, that is, they exist in the mind, or are perceived by it, as truly as the ideas of its own framing. The ideas of Sense are allowed to have more reality in them, that is, to be more (1) strong, (2) orderly, and (3) coherent than the creatures of the mind; but this is no argument that they exist without the mind. They are also (4) less dependent on the spirit or thinking substance which perceives them, in that they are excited by the will of another and more powerful spirit; yet still they are ideas, and certainly no idea, whether faint or strong, can exist otherwise than in a mind perceiving it.

But is there not a danger of solipsism here? If everything exists in the mind, how can we have a common reality?

Ponder if you will….

How would Mr. Ockham’s razor explain your experience of, say, a keyboard?

Is it simpler to say that you have an image of the keyboard in your mind and the keyboard also exists outside your mind, OR to simply say I know the keyboard as reality in my mind only?

…it will be objected that by the foregoing principles all that is real and substantial in nature is banished out of the world, and instead thereof a chimerical scheme of ideas takes place. All things that exist, exist only in the mind, that is, they are purely notional. What therefore becomes of the sun, moon, and stars? What must we think of houses, rivers, mountains, trees, stones; nay, even of our own bodies? Are all these but so many chimeras and illusions on the fancy? To all which, and whatever else of the same sort may be objected, I answer, that by the principles premised we are not deprived of any one thing in nature. Whatever we see, feel, hear, or anywise conceive or understand remains as secure as ever, and is as real as ever. There is a rerum natura [i.e., things of nature], and the distinction between realities and chimeras retains its full force….

But remember, for Berkeley to exist is to be perceived by a mind (esse est percipi) and it is the mind of God that gives objectivity to the contents of our perceptions. All Berkeley is saying is that those objective objects are mental and not physical in nature.

Vide sect. lxxxiv.]–I do not argue against the existence of any one thing that we can apprehend either by sense or reflection. That the things I see with my eyes and touch with my hands do exist, really exist, I make not the least question. The only thing whose existence we deny is that which philosophers call matter or corporeal substance.    (Berkeley, A Treatise Concerning the Principles of Human Knowledge)

Berkeley, then, claims to be denying not the reality of the sun, moon, or stars. He “only” claims that their natures have been misunderstood; they are ideas, and they are not composed of matter. And we can tell that they are real, and that the cogitations of our imagination are not, because our ideas of e.g., the sun, moon, and stars are more “strong, orderly, and coherent” than, say, MacBeth’s hallucination of a dagger before him.

Another common question for idealism – and the final one that we will consider here – involves the existence of objects that are not presently being perceived. Surely the sidewalk in front of my house still exists, even when nobody is walking on it or driving by it or looking out of their window at it or thinking about it at all, right? Surely it would be too miraculous to suppose that it stops existing during such times, and then comes back into existence exactly as it had been as soon as someone looks at it. Right?

Berkeley’s version of idealism claims that the sidewalk does still exist because even when no humans or animals perceive it, God always does. In this way, Berkeley gives God a surprisingly central role in his metaphysics. It is the Mind of God that assures us that our perceptions are indeed quite real, for to be real is to be perceived by a mind.



Works Cited

Berkeley, George. A Treatise Concerning the Principles of Human Knowledge, Project Gutenberg, 7 Mar. 2002, https://www.gutenberg.org/files/4723/4723-h/4723-h.htm.

Hume, David. “Appendix I.” An Enquiry Concerning the Principles of Morals, The Gutenberg Project, 12 Jan. 2010, https://www.gutenberg.org/files/4320/4320-h/4320-h.htm.

“The Ockraz Logo.” Wikimedia Commons, 29 Nov. 2008, https://commons.wikimedia.org/wiki/File:The_Ockraz_Logo.jpg. Accessed 17 May 2022.

Stoljar, Daniel. “Physicalism.” Stanford Encyclopedia of Philosophy, Stanford University, 25 May 2021, https://plato.stanford.edu/entries/physicalism/#NumbAbst).


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PPSC PHI 1011: The Philosopher's Quest by Daniel G. Shaw, Ph.D. is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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