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René Descartes was famous for his belief in substance dualism – that the mind and the body are two separate substances and, in his view, they interacted with each other in the pineal gland. Forgetting the nonsense of the pineal gland being where it all happens, the general notion of substance dualism is widely held by many around the world.

Of course, in the religious world, this is important for many ideas of supernaturalism and how it all works, certainly regarding a soul. Believing in the naturalist Carradine, substance dualism can be an attractive position.

However, how would it really work? In reading The Big Picture: on the Origins of Life, Meaning, and the Universe Itself by Sean Carroll, I was reminded of how Descartes was challenged by Princess Elisabeth of Bohemia, who was herself a pious lady of a Calvinist tradition.

Here is an excerpt from the book that references the conversations that took place by letter between the two people way back when. It must be noted that there would now be a lot more points to enter what’s Elisabeth herself documented, but she did a fairly good job at showing how weak Descartes’s position was. Elisabeth’s quote is in italics:

How can the soul of a man determine the spirits of his body so as to produce voluntary actions (given that the soul is only a thinking substance)? For it seems that all determination of movement is made by the pushing of a thing moved, either that it is pushed by the thing which moves it or it is affected by the quality or shape of the surface of that thing. For the first two conditions, touching is necessary, for the third extension. For touching, you exclude entirely the notion that you have of the soul; extension seems to me incompatible with an immaterial thing. This is why I ask you to give a definition of the soul more specific than the one you gave in your Metaphysics.

It’s a question that cuts to the heart of the mind/body split. You say that mind and body act on each other, fine. But how, exactly? What precisely happens?

It’s not simply a matter of “We don’t know this part of the story, but we’ll figure it out eventually.” Elisabeth was presumably not a physicalist, someone who believes that the world is made purely of physical stuff. Not many people were in 1643. She was a pious Christian, and most likely had no trouble believing there was more to life than the immediately apparent world. But she was also scrupulously honest, and could not understand how an immaterial mind was supposed to push around the material body. When something pushes something else, the two things need to be located at the same place. But the mind isn’t “located” anywhere—it’s not part of the physical plane. Your mind has a thought, such as “I’ve got it—Cogito, ergo sum.” How is that thought supposed to lead to the body lifting a pen and committing those words to paper? How is it even conceivable that something with no extent or location could influence an ordinary physical object?

Descartes’s initial response was at once both fulsomely flattering and somewhat patronizing. He wanted to remain in the princess’s favor, but at first he didn’t take her question all that seriously, offering a halfhearted suggestion that “mind” was somewhat like “heaviness,” though not really. His argument was the following (roughly paraphrased):

  • We want to know how an immaterial substance such as the soul can influence the motion of a physical object like the body.
  • Well, “heaviness” is an immaterial quality, not a physical object itself. And yet we often speak as if it has an effect on what happens to physical objects—“I couldn’t lift that package because it was too heavy.” That is, we attribute causal powers to it.
  • Of course, he quickly notes, mind is not exactly like that, because mind actually is a separate kind of substance. Nevertheless, perhaps the way the mind influences the body is somehow analogous to the way we say heaviness influences objects, even though one is a true substance and the other is not.

If you’re confused, you should be, since Descartes’s story makes no sense. Ironically, though, it’s close to correct. To a poetic naturalist, “mind” is simply a way of talking about the behavior of certain collections of physical matter, just as “heaviness” is. The problem is that Descartes is nobody’s naturalist. His burden was to explain how something nonphysical could influence something physical, and he proffered an explanation that utterly failed to do so.

Elisabeth was not impressed. In her subsequent letters she continued to press him on the issue, explaining that she knew perfectly well what heaviness was, but couldn’t fathom how it was supposed to help her understand the interactions of physical bodies and immaterial minds. She asks why a mind that is completely independent of the body could be so affected by it—why, for example, “the vapors” are able to affect our capacity for reasoning.

Descartes never offered a satisfactory answer. He believed that the mind’s relationship to the body was not like that of a captain to his ship, with the mind pushing around the material object; rather, the two were “tightly joined” and “mingled together.” And that mingling occurred, he hypothesized, in a very particular anatomical location: the pineal gland, a tiny part of the vertebrate brain that (we now know) produces the hormone melatonin, responsible for our sleep rhythms. He focused on that specific organ because it seemed to be the only part of the human brain that was unified rather than split bicamerally, and he believed that the mind only experienced one thought at a time. Descartes suggested that the pineal gland was a physical object that could be moved both by the “animal spirits” of the body, and by the immaterial soul itself, serving to mediate influences between the two.

The suggestion that the pineal gland serves as “principal seat of the soul” never really caught on, even among thinkers who were otherwise sympathetic to Cartesian dualism. People continued to try to understand how the mind and body could interact. Nicolas Malebranche, a French philosopher who was born just a few years before Elisabeth and Descartes began their correspondence, suggested that God was the only causal agent in the world, and that every mind/brain interaction was mediated by God’s intervention. As Isaac Newton later noted in a discussion of vision, “To determine by what modes or actions light produceth in our minds the phantasm of colour is not so easie.”

Carroll, Sean. The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (Kindle Locations 3189-3192). Penguin Publishing Group. Kindle Edition.

Not only do we have the problem of what the soul actually is and does, as I have previously expounded, but we also have the problem of how it works and how it interacts with the physical body. It’s not good enough (at least for me, anyway) to say, “Just because we don’t know how it works, it doesn’t mean that it doesn’t work!” I think that when the whole thesis for the soul is built on such flimsy foundations and when no evidence is really offered for its existence at all, then to show that there is no evidence of how it could possibly work is also an important part of the jigsaw.

And this jigsaw clearly shows a picture of a physical body that exists without a soul. Now, this brings us onto conversations of consciousness, and it also sets up the premise that the mental of consciousness either entirely supervenes on the physical or is indeed physical itself in some manner.

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Jonathan MS Pearce

A TIPPLING PHILOSOPHER Jonathan MS Pearce is a philosopher, author, columnist, and public speaker with an interest in writing about almost anything, from skepticism to science, politics, and morality,...