Friday, October 19, 2012

How mind-body interaction in Aquinas is not subject to Cartesian problems

So, Descartes famously establishes a mind-body dualism in his Meditations on First Philosophy.  He divides substance into two species: res extensa and res cogitans (extended stuff and thinking stuff).  Res extensa is material, extended, divisible, etc.  Res cogitans is immaterial, non-extended, simple, etc.  A human being is primarily a cognitive substance united to an extended body.  But given the total diversity of res cogitans and res extensa, it becomes very difficult for Descartes to explain how the two could interact causally.  How can changes in the body be communicated to the soul (e.g. sensation), and how can motions of the soul be communicated to the body (e.g. in willing)?  What mediates this interaction?  Descartes realizes this is a problem, but his solution is famously inadequate.  He says that the locus of interaction between body and soul is the pineal gland in the brain, and the two interact by the exchange of pressures on a certain membrane.  Now, obviously this doesn't work because by claiming that the soul can exert pressure on a membrane he is effectively predicating a spatial, material action of a non-spatial, immaterial substance.  And so this begs the question: how can the soul move the pineal gland in the first place?  And we're back where we started.  This problem continues to haunt mind-body dualism.  (Thus the need for Cartesian duellists to defend the honor of the system. Dylan Morris)

Now, in Aquinas, there is at first glance a similar duality between soul and body: the body is material and the soul is subsistent and immaterial.  Some actions of the human are referred to the body (e.g., digestion, imagination), and some to the immaterial soul (e.g. willing, intellection).  In fact, a lot hinges on the fact that certain functions of the human person cannot be cogently described as acts of a physical organ (proofs of the immateriality and hence immortality of the soul, notably).  But consider the following situation.  Suppose you're walking in the woods and you come across an unusual flower, one that you haven't seen before.  According to the Thomist, the process by which you're able to know proceeds causally as follows: first the flower exists and has its own form, but then by the action of the sun illuminating the flower, it is capable of communicating its form as a visible image to your eye.  The image on the retina which conveys the form is then received by the common sense, and this datum elicits a phantasm in the imagination.  All of this, according to the thomist, happens in physical organs (your eye and brain).  The next step involves a shift: the agent intellect renders the phantasm intelligible and abstracts universal content from it, which is then impressed on the passive intellect, by which you are able to apprehend and make judgments about the universal attributes of the flower.

The problem one might pose is this: in the interaction between the corporeally situated faculty of imagination and the immaterial/spiritual faculty of understanding, there seems to be some leap.  What was in the person corporeally is suddenly shifted over to something incorporeal: a material fact has some effect on an immaterial thing.  This seems to cause the same difficulties as Descartes faces in his dualism: how can we explain the communication of motion from an immaterial substance to a material one and vice versa?  The thomist might weakly reply that the immaterial soul is the form and life of the body, and thus has a substantial unity with the corporeal functions of the person that Descartes ghost-in-machine anthropology doesn't allow for.  But even so, it remains true that the intellectual faculties are not situated in an organ, and thus it seems impossible to explain interaction between the material and the immaterial processes here.

The solution to the problem is to remember that all material beings are, according to St. Thomas, a composite of matter and immaterial form.  The form of a material thing is not subsistent, i.e., it does not have its own being apart from the composite in which it exists, but it can be communicated to another thing inasmuch as that thing is capable of receiving it.  (Whatever is received is received according to the mode of the receiver.)  So, for example a piece of film is capable of receiving the form of a flower in a certain way, or a piece of canvas in another way.  What is communicated here isn't a bit of the flower, but its form: you don't have to cut off a petal to take a picture of the flower, but even so something of what the flower is comes to be in the film.  What is communicated is immaterial.  But in this case the cartesian problem vanishes, because there is no longer a communication of material stuff to immaterial stuff: the interaction is not causal after the fashion of billiard balls bumping into each other (how could a brain bump into a soul? what would mediate this interaction?), but in the broader understanding of causation as the communication of forms from one agent to another.  In the line of causation from the form of the flower subsisting in it materially to the image in the eye and the phantasm in the brain to the abstract from in the intellect, what is communicated is immaterial (the form) and that to which it is ultimately communicated (the passive intellect) is also immaterial.  Hence hylomorphism seems to do away with one of the most crippling problems of Cartesian mind-body dualism.


  1. I think this argument only solves part of the interaction problem, though one is not obliged in a single adventure to slay every dragon.

    A seal imparts an image in wax, and an immaterial form is communicated to the wax. And so (as you say) the communication of immaterial forms by material means is not unusual. But the wax does not know the image on the seal.

    So also, the common sense (which I suppose is in the sensory cortex or some part thereof) does not know the sensibles I encounter. To clarify: a brain, understood in se and not as an integral part of an animal endowed with sense, does not know, but merely receives external impressions in the form of patterns of neural excitation. A stimulated neuron is here analogous to a light-sensitive molecule in a piece of film -- and therefore also analogous to the malleability of wax.

    But in this, a brain (or a piece of film or wax) has received another's form in its own proper way: the brain as a neural excitation, the wax as a physical shape, etc. But if to know is to be another as other, this is not yet adequate for knowledge.

    How does the passive intellect acquire these forms from the common sense? The common sense has been made another as self -- as a result of which the mind has been made another as other. There is still a mystery of interaction here. And I don't know what analogues we have (cf the wax and the film) to help us understand the activity of the passive intellect.

    1. I think what we can say is that the soul is united to the body as its form, but as a form with its own proper operations and subsistence. Any form in the body is therefore in the soul, but because the soul is not merely the form of the body but something else as well, the operation on forms abstracted from things and received in the sense or the imagination is still possible. That the passive intellect is said to receive forms like wax doesn't really seem to matter. What matters is that the intellect subsists immaterially and is in potency to receive intelligible forms. The wax metaphor is only meant to help us understand how one thing can receive the form of another without changing in its species. I think you're making another point but my head is too foggy to tease it out. Feel free to say more.

    2. I think the point I was trying to make was that we have no analogues for knowledge -- there is nothing in our experience that is like knowledge.

      We see the communication of forms around us constantly, but except in our own minds we only see this happening through physical change. If we look for an example of something that becomes another as other, our mind is the only example we see. And so when it comes to this point (more or less: the immateriality of mind), any analogy we might make to other kinds of formal communication is useless.

      Maybe we are not permitted to know how the passive intellect works.

    3. It's true that there can't be an empirical analogue for knowledge, but I think we can know what knowledge is like simply because we observe it constantly. The more we know, the more we learn, the more we witness the advent of understanding in the mind, the impression of forms upon the intellect. If the prime analogate is always to be something in sense experience, then the mind must remain a mystery. We are much closer to ourselves, though, than we are to bees or trees, even though the passivity of the intellect dictates that we only gain self-knowledge by observing our understanding of bees and trees.

  2. "Any form in the body is therefore in the soul."

    A Saracen comes along and chops off my leg. The form of my body has been changed, but my soul has not.

    A butterfly of a species I have never seen comes before my eyes, leaving an impression on my retinas -- this causes a change in my soul.

    A tattooist leaves an impression on my skin -- this does not cause a change in my soul, right?

    What I'm trying to get at (I think):

    Immaterial forms are communicated all the time. This isn't strange at all and we can think of all kinds of uncontroversial examples in physics.

    In sensation an immaterial form is communicated physically but also intentionally.

    In intellection an immaterial form is communicated intentionally but not physically.

    There is a sense in which sensation and intellection happen "mechanistically" (not, however, in a way reducible to quantity). If a sensible comes before an organ of sense, it is sensed. If a phantasm comes before an intellect, it is known.

    What can we say about these mechanisms?

  3. I think you'd be hard pressed to say that your soul has not changed as a result of losing your leg.

    As for the tattoo, quidquid recipitur... How are you receiving the form? You're receiving it as pigment in flesh. Does it affect the form of your body? Yes. Does it affect your soul? Yes. Is it in the soul cognitively by virtue of being imprinted on your flesh? No. If the point is to find out what distinguishes the reception of a tattoo into the flesh and the reception of an image in the retina, I think we could say that it's the power of the organs doing the receiving that differs.

    We can say a lot about these mechanisms, but I'm not sure what you want to be said about them. Be more specific.

  4. In brief: it strikes me as a kind of miracle that physical actions of an ordinary type can give rise to cognition.

    Yet this is part of the everyday life of men and animals, and so is surely not miraculous.

    What then is going on? What is it that happens within me that allows a physical change to become sense? It's all well and good to say that it happens because of a power of the organ: vermöge eines Vermögens, as it were.

    "Quicquid recipitur" means the same thing for an eyeball as for a camera. (Did you ever get to play with sheeps' eyes in middle school, using them as a kind of lens?) But an eye gives rise to sense.

    Why are some parts of the body enabled to affect the soul cognitively? How do they pull it off, when they seem to be simple and certainly uncognitive objects in themselves?