Metaphysics

A Critique of Descartes’ Interactionist Dualism

As Jaegwon Kim notes, what distinguishes Descartes’ philosophy of mind from his contemporaries like Leibniz and Spinoza is his “imminently commonsensical belief” that minds and bodies causally interact with each other (Kim, 34). Despite its intuitive simplicity, this is where the main problem in the Cartesian dualism arises. This paper explains and examines the problem of mind-body interaction and at the same time accesses the plausibility of objections to Descartes’ interactionist substance dualism.

Prior to examining the problem of interactionism, it is crucial to note (briefly) what Descartes meant by “substance” and how does he argue for substance dualism. Descartes maintains that, firstly, a substance is something in which properties “inhere”, i.e. it is what has properties. Secondly, that it can exist by itself, i.e. without the aid of any other substance (Kim, 33). Then with his famous “cogito” argument, Descartes argues that they are substances of two fundamentally different kinds in the world, mental and material substances. The essential nature of a mind is to think, to be conscious and to engage in other mental activities while the essential nature of a body (matter) is to have spatial extension and be located in space (Sober, 272). By Leibniz’s Law [1], Descartes maintains that his argument is established that mental and material substances are distinct by the virtue of their properties such as extension, thinking and divisibility. As Sober points out, plausibly, there seems to be no reason to accept that mind is indivisible or lacks extension unless one already believes that the conclusion (dualism) is true (Sober, 273).

Another problematic issue in Descartes’ argument for substance dualism is his assertion that conceivability entails possibility. As Kripke refutes, conceivability or imaginability doesn’t necessarily entail possibility. He argues that, “there are many epistemic possibilities which are imaginable because they are epistemic possibilities, but which are not real possibilities (Robinson, 29).” Finally, as stated above, the most daunting challenge for a Cartesian dualist is to defend the position that minds and bodies causally influence and interact with each other. Now, the question is whether this interactionist thesis is compatible with the radical dualism of minds and bodies. In other words, the problem is how does this commonsensical view of mind-body interaction works within the Cartesian dualism of non-spatial immaterial minds and material substances in the space-time world. Now I turn to explain the problem of interactionism and analyze the plausibility of objections offered respectively.

Assuming that minds and bodies are distinct substances as such, the Cartesian dualists are to explain and defend that it is possible to maintain their dualist ontology of minds and bodies as distinct substances yet at the same time causally interact with and influence each other. This two-way causal traffic between mind and body seems intuitively obvious and commonsensical, but what makes this problematic for Descartes is his dualist theory of minds and bodies as fundamentally distinct and different kinds of substances. Descartes’ theory of dualism obscures the “how” of the interaction, because he seems to fail in offering a reasonable account of the interaction (Armstrong, 18). Since the immaterial substance (mind is immaterial) not extended in space as the body is, the mind is not in contact with the body. Descartes, clearly, recognizes this problem of causal interaction as he attempts to present his arguments by suggesting the pineal gland as the spot where the interaction between immaterial mind (in the form of different mental activities) and material body takes place; he calls it the ‘seat of the soul’ (Kim, 47). In the Sixth Meditation, Descartes explains that mind is not immediately affected by all parts of the body, but only by the brain. He posits that the brain receives the neural signals from parts of the body after which it (brain) gives the mind its signal of having a certain sensation (for instance, pain in the left arms) in the particular region and this sensation stimulates the mind to do its best to get rid of the cause of pain (Kim, 47). This is problematic because the causal relationship between mind and body is still mysterious and Descartes fails to explain or give an account of the interaction that can be tested empirically. Armstrong argues that, first of all, Descartes’ suggestion for pineal gland as the center of the causal interaction is an ingenious suggestion as it can be empirically disproved. Secondly, there is still this problem of missing link between mind and body, because physical scientists maintain that physically closed systems conserve energy and the total energy in a system (the domain of physical phenomena is causally closed) doesn’t alter (Armstrong, 19). Now, the problem arises because this sort of causation (Descartes’ explanation) lacks any parallel theory of causation if there is any causal relation in it in the first place.

Princess Elisabeth of Bohemia, who was a student of Descartes’s writes to him and challenges him to explain this problem. To explain her problem with an example, for any movement to be determined like person X’s decision to raise his/her arms, it seems that this always come about by X’s arms being propelled –by a kind of impulse to raise his/her arms to touch the ceiling. Now first two conditions of arms and touching the ceiling involve contact, but the third condition of impulse also necessarily involve extension; but Descartes rejects extension to mind or mental activities. Princess Elisabeth argues, “…but you exclude extension from your notion of soul, and contact seems to me to be incompatible with a thing’s being immaterial (Armstrong, 19).” Her problem is a legitimate one to raise, but as Armstrong points out, here we are working with the assumption that causality needs contact; and it remains true that we do not have other model or theory of causality to explain how ‘spiritual happenings give rise to a brain happening and vice versa’ (Armstrong, 20). Kim translates Princess Elisabeth’s problem into modern terminology and explains its incompatibility with the basic laws of physics, for instance, flow of energy in a physical system. A perfectly legitimate question to ask is: how could there be energy flowing from immaterial mind to a material body? Or how could anything “flow” from something metaphysical (out side space) to something in space? Descartes replies to Princess Elisabeth by appealing to a “primitive” notion of mind-body union –as a fundamental notion that is intelligible in its own right and cannot be explained in terms of other basic notions (Kim, 49). This clearly didn’t satisfy Princess Elisabeth as she responds by saying that instead of allowing the possibility of mental causation, she would rather accept materialism concerning the mind (Kim, 49-50).

Well, it seems that all these problems we encounter are due to a conflict between interactionism and some basic physical laws. An interesting counterargument is that physical laws such as conservation principle are not ubiquitous in physics. Robins Collins argues that the energy is not conserved in general relativity or quantum theory, why then, should we insist on it in mind-brain interaction (Robinson, 17)? As Robinson points out, these debates concerning interactionism take place in the context of the assumption that it is incompatible with the world’s being “closed under physics”, but it is not justified if causal overdetermination[2] of behavior is possible (Robinson, 16). The problem with physical closure seems to disappear if physical laws are indeterministic. As quantum theory seems to assert, if it were indeterministic, then any interaction from outside the system would not violate the physical laws (Robinson, 17). This is still a highly contested area even in physics and no further comments can be made confidently, but this is reasonable point to consider.

Epiphenomenalism holds the view that brain causes mental events, but those mental events cannot cause behavior, it is a one-way causation. This approach clearly avoids the problem of physical closure. But epiphenomenalism has some serious problems. First of all, it is highly counterintuitive. What could be more apparent than that it is fear of being hit by the motorbike and I run across the road swiftly when I saw it speeding towards me? It seems plausible to maintain that there are conscious states that clearly modify our behavior (Robinson, 19). Epiphenomenalism reduces humans to mere spectators of their lives; they can only watch what happens but cannot affect it. If interactionism is mysterious for its failure to explain how immaterial and material things interact, epiphenomenalism faces a similar problem of explaining how material things (brain) cause something immaterial like desire or belief. It seems equally mysterious.

Descartes’ contemporary, Leibniz, maintains that no coherent sense can be made of Descartes’s notion of mind and his interactionism is impossible. Leibniz attributes to God to have initiated the union of mind and body in such a way but they don’t casually interact. His position is called parallelism and it denies any causal interaction between mind and body. It posits that mind and body run in harmony with each other. Leibniz believes the union of mind and body is a “pre-established harmony” set in place by God and he even thinks that God might as well set things up so they always appear as if they are interacting (Kulstad & Carlin, 8-9). Others like Malebranche argue that both mind and body are causally ineffective, God is the only true cause and for every occasion God intervenes as the cause both perception and behavior (Robinson, 21). It is obvious that the problem with Leibniz and Malebranche stem from the need to postulate a “transcendental hand” to intervene. Their positions are incredible outside their theistic frameworks. Interactionism, despite its difficulties, it still seems to remain the most plausible theory of mind-body interaction available to a dualist.

Work Cited

Armstrong, D.M. The Mind-Body Problem: An Opinionated Introduction, Westview Press, 1999.

 Sober, Elliott, Core Questions in Philosophy: A Text With Readings, 4th Edition, Upper Saddle River, 2008.

Kim, Jaegwon, Philosophy of Mind, 3rd Edition, Westview Press, 2011.

Kulstad, Mark and Carlin, Laurence, “Leibniz’s Philosophy of Mind”, The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.)

Robinson, Howard, “Dualism”, The Stanford Encyclopedia of Philosophy (Winter 2012 Edition), Edward N. Zalta (ed.)


[1] Leibniz’s Law or Indiscernibility of Identicals states that: if x = y, then x has all the properties y has (and vice versa).

[2] Overdetermination is a phenomenon whereby a single observed effect is determined by multiple causes at once, any one of which alone might be enough to account for (“determine”) the effect.

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