This article was last modified on September 28, 2010.

Mary the Materialist: Right and Wrong

Rocco J. Gennaro has a short book on the mind and brain that features a dialogue between three characters: Mary the Materialist, Dave the Dualist and Steve (unidentified, but apparently the Skeptic). Dave and Mary argue and defend their sides, while Steve questions them both without fully committing to either perspective. As Gennaro seems to make Mary’s arguments the stronger of the two, I assume this is his position. As such, I wish to outline the argument for materialism from Mary, in her own words. All unidentified quotations are from the book “Mind and Brain”.

Mary’s General Position: Opening Statements

There are varying kinds of materialism. In philosophy, there are varying kinds of every belief, and that is why defining our terms is of the utmost importance. What kind of materialist is Mary?

She says, “I am a materialist about the human mind… all mental processes are simply brain processes or states… the mind is the brain.” Again, later, she says, “every mental state is a physical state or, more specifically, a neural state or brain process.” Simply put, Mary denies the existence of the mind as being a separate entity from the brain: all mental events can be described in the words of the physical brain, if only we had enough knowledge. When asked if she is a “token-to-token” materialist, Mary says, “I guess so.”

(I don’t want to bog this article down with technical terminology. For those who want it, here is the Wikipedia entry explaining the token physicalism Mary apparently accepts: “Token physicalism is synonymous with property dualism. Token physicalism states ‘for every actual particular (i.e., object, event or process) x, there is some physical particular y such that x=y’. This does not entail nor is entailed by supervenience, although if supervenience is true, it does not necessarily rule out token physicalism. The difference between supervenience and token physicalism is simple; token physicalism states that for every mental particular there is a physical particular to which it is identical, while supervenience physicalism states that set A (e.g., mental properties) cannot change unless set B (e.g., physical properties) changes as well. (i.e., A supervenes on B). As the name suggests, this is a dualistic conception of reality that does not discount the option of physical properties also having non-supervened mental properties. Supervenience physicalism rules out this possibility.” Now you’re probably wondering what supervenience is… and that is why I will avoid technical language, because it will only lead to more definitions. )

Why Materialism?

Anyone with a background in philosophy, especially philosophy of mind, will find Mary’s materialism pretty standard and even common. But reasons for believing in materialism may vary. What are Mary’s reasons?

I think there are three reasons she lays out. The primary reason Mary accepts materialism is because of her acceptance of science and scientific knowledge. For example, Mary believes in evolution and “only materialism makes sense in this context.” If evolution is true, for example, where between apes and us did the mind/soul come along? (Mary tends to use the mind and soul interchangeably, as explained below in the immortality section.) Of course, theologians may attempt to answer this for her. Catholics officially accept evolution, yet also accept a soul (and presumably a mind). So whether or not these things can exist side-by-side, many people accept that they do.

The second reason, and the one I find most compelling, is her use of the principle of simplicity (also known as Occam’s Razor): “if there are two theories both of which explain the same number of observations or facts, then we should accept the one that posits fewer objects or the one that is more simple.” She uses it to support her view when she says, “Even assuming… dualism could explain as much about the mind as materialism, why accept the theory that posits an extra mysterious non-physical entity when the brain will do just as well?” The principle of simplicity is an effective tool for eliminating supernatural explanations for things that could also be explained physically. And Mary has a valid point: assuming that materialism could explain everything about the mind, dualism would be unnecessary. Her only problem is that at this point, materialism may not be able to explain everything.

Third, she offers what I call the “wishful thinking” belief. Mary says, “If materialism is true, it is at least possible to explain mental functioning in terms of the brain.” She believes that “materialists have a chance at scientifically understanding the mind, whereas dualism leaves us with no hope at all.” There is reason to have hope. Asshe says, “we have a good deal of knowledge about the way neurons work and how some brain activity is correlated with certain types of mental states. For example, many parts of the brain are clearly linked to pain, memories, and visual experiences.” And this relates back to her second point — if scientists learn enough, and could explain it all physically, dualism would become obsolete. But to say that materialism, if true, could use science to explain the brain, and then accept it on those grounds is a leap: you cannot assume something is true because you like the possible outcome.

Let me try to rephrase her logic: “If Heaven is true, it will at least be possible to be rewarded for our good deeds in life.” Yes, we would all love to be rewarded for our good deeds, but this desire does not in any way make the idea of Heaven any more possible. We may also want science to be able to explain the mind, but we have to accept the possibility that maybe it never will.

Against Immortality

Whether or not materialism is true, Mary uses this belief as a foundation for other beliefs. The first of which is, “I don’t believe in immortality”.

As she says, “if the mind is the brain, then at death it must cease to exist and must eventually rot away with the rest of the body.” Mary actually goes on at some length about the idea of an immortal mind, and why she sees it as bogus: “When we describe what is supposed to be immortal, it sounds like what we mean by the mind. I defy you to explain any coherent distinction between the mind and the soul. As a matter of fact, these terms are used interchangeably in many historical texts, and, for example, both English words are used to translate the single Greek word ‘psyche‘. Granted, sometimes the term ‘soul’ carries a more theological connotation, but it doesn’t follow that the words ‘soul’ and ‘mind’ refer to entirely different things.”

If she is correct that the mind and soul are the same, then her reasoning here is sound: if materialism is true, there seems no reason to accept immortality. The body clearly does die.

Against God

Mary next makes the bigger religious claim when she says, “I do not believe in God.” But her reasoning is suspect, because she justifies this lack of belief by asserting that “if our death really is the end, then why believe in God?” This is somewhat problematic.

If Mary says she does not find God to be necessary, that is a very scientific argument. Stephen Hawking recently released a book making this claim, and he is relying on physics, the most materialistic of the sciences. Yet, he has been clear to say that even though the universe does not “need” a god, such a thing cannot be disproved. Mary is entitled to not believe in God, but to base this on the reason that we are not immortal seems absurd. There is a possibility that God created people to be mortal and nothing more. Theologians have no problem viewing mere animals as mortal; why should God’s existence suddenly come into play if we lump people in to this same group?

For Non-Material Entities

Perhaps the most interesting thing about Mary’s materialism, and any sort of materialism, is trying to explain the existence of non-material things.

Mary asserts, “I believe in other nonphysical entities” and then clarifies that “I believe in numbers.” As she says, “Numbers are nonphysical and eternal.” And then, “There are certainly some numbers which have never been written down anywhere. Don’t they exist now? Of course.”

What are we to make of this? Surely, numbers exist. But how to explain them in physical language? Colors can be described as a certain wavelength of light, but what are numbers? If we have two apples, it is plain to see “two” as a physical property of the apples. But what of numbers much larger, or strange numbers like pi? Pi exists. But how would a materialist explain it? Mary seems to simply accept this, but is that good enough. Can a materialist claim that all things are physical, and then believe in nonphysical entities?

(Others have asked how materialists can discuss and understand concepts like “Zeus” or “Unicorns”, but that is beyond the scope of this article.)


As a materialist myself, I am sympathetic with Mary’s perspective. Yet, her logic leaves something to be desired. I do find materialism to be more likely true, particularly with the growing scientific evidence, but Mary’s absolute certainty seems misguided and a poor personality trait for a philosopher to have. One must always keep an open mind and be willing to change positions as facts arise.

I feel Mary fails to adequately “prove” materialism, and her denial of God rests on shaky ground. She then admits to accepting non-physical numbers. While it seems clear one must accept them (numbers obviously exist), she makes no attempt to explain how any non-physical can exist in the physical world.

In short, Mary’s beliefs strike me as being on the right track, but in need of some serious refinement.


Gennaro, Rocco J. Mind and Brain: A Dialogue on the Mind-Body Problem. Hackett Publishing, 1996.

Also try another article under Philosophical
or another one of the writings of Gavin.

One Response to “Mary the Materialist: Right and Wrong”

  1. Joseph Says:

    Numbers, being immaterial, a concept, exist as a pattern of neural connections and pathways. Smarter people grasp a lot more about them than the less smart. Even simpler lifeforms can have an rudimentary yet instinctive grasp of numbers that are passed down to offspring because it aids survival of the species. Humbly yours.

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