The 'How Does it Work?' Objection to Dualism: #1 Different Substances

"How can the mind move the body?" This is one of the most familiar rhetorical questions for dualists about the mind. It is rhetorical since it is meant to make the point that dualists can't explain how the mind moves the body, and that this is a problem for dualism.

One problem is that this question is highly ambiguous. This is reflected in the literature by the fact that the phrase 'interaction problem' is used by different philosophers to refer to quite different arguments. When someone poses this question there are multiple objections someone could have in mind. Someone could be saying it's in principle impossible for dualists to provide an explanation. Or they could simply be saying there is no plausible candidate for an explanation. Or they could be saying it's very likely that non-physical causation never happens in our world. Moreover, they could pose these arguments for different reasons. So it's important to get clear on which objection we're talking about. I'll try to distinguish a few ways we can formulate this objection and then reply to these objections. In the first post I'll identify and respond to what I call the 'Different Substances Objection'.

#1: The Different Substances Objection

First, one might object that, since mind and matter on a dualist view are completely separate there is no way for them to interact. Everything is mental or material, but not both. So there could not be interaction between the two. The problem is that it's not generally true that entities from two mutually exclusive categories cannot interact causally. Everything is either a proton or a non-proton for instance, but that doesn't mean protons and non-protons cannot interact.

Another very similar form of this objection is that mind and matter are different types of substances, so the two cannot interact. Or a more particular version of this objection would say that the mind is non-extended and the body is extended, so the two cannot interact. One problem is that, in general, it is false that in order for substance x to interact with substance y of kind F, x must itself be of kind F. You don't need to be a human to interact with a human; you don't need to be a proton to interact with a proton; etc.

A final and more sophisticated way to formulate this version of the objection is that since mind and matter share no intrinsic properties, the two cannot interact. First of all, the main premise of this argument is not true. Being a substance is common to both our minds and our brains. Having causal powers is intrinsic to both (note: one need not presuppose causation between mind and body occurs to hold this is true, since one should at least admit mental to mental causation). They both have the property of having metaphysical components. Of course, they don't share any intrinsic physical properties (since the mind does not have physical properties). But at least some of what I have listed are plausibly intrinsic properties.

More importantly though, why do two substances have to have common intrinsic properties to causally interact? This principle would have to be motivated by some more general theory where two substances' having common intrinsic properties P1, ... , Pn plays a relevant role in their ability to engage in causal interaction. In particular, to be relevant, if x causes change C in y in some way, then these P1, ... , Pn must contribute some causal influence to C. I'm not sure how else one would motivate the main premise here.

But we can imagine situations where the common intrinsic properties of agents plays no relevant role in the interaction of the two. For instance, even though a Jedi and a table share the intrinsic properties of having mass or volume or other physical properties, when the Jedi uses the Force he doesn't in any way seem to take advantage of his having mass or volume or his other physical properties. Of course, this is just an imaginary situation, but it seems perfectly coherent and thus there can be no a priori reason for thinking common intrinsic properties must play a relevant role in all causal interaction (of course, this isn't to deny that sometimes they do).

In fact, there might be some cases which are actual counterexamples. For instance, consider the EPR paradox cases from quantum mechanics. Suppose you have a source emitting an electron-positron pair in a state of quantum entanglement, where the spin of each is anti-correlated with the other. In other words, if electron e has upward spin then p has downward spin, and if e has downward spin then p has upward spin. Suppose moreover you have two observers A and B in different locations who can measure the spin of the particles along some axis Z, and e is sent to A while p is sent to B. If A measures e as having an upward spin then B will measure p as having a downward spin with 100% probability. On the other hand, if A measures e as having downward spin, B will measure p as having upward spin with 100% probability. Since experimentation and Bell's Theorem rule out local causal explanation here, and supposing there is causal interaction between e and p, there must be some causation here where local intrinsic properties such as mass, volume, velocity, etc. do not play a role in the causal interaction.

Of course, the particles do share the intrinsic properties of having spin. But it's not e's simply having spin which contributes causal influence to p's particular spin; rather, it is e's having an upward or downward spin which does so. For what the spin of p is depends on the actual spin of e. So it isn't clear that we can identify some common intrinsic property of e and p where's e's having this property causally influences p's having a particular spin.

This seems to me to be an adequate reply to the different substances objection. In the next post I'll talk about what I call the 'No Mechanism Objection', which poses the problem that there seems to be no familiar model which could make causal interaction between the mind and the body intelligible.

An Issue With Metaphysical Reduction

Take a fact F. In general, what does it mean to say that fact F metaphysically reduces to fact F'? Note I am speaking of metaphysical reduction as opposed to conceptual reduction. First of all, the latter has to do with concepts and propositions rather than facts. For example, when we say that being a bachelor just means being an unmarried male, or when we say the proposition that Alfredo is a grandfather just means that Alfredo is the father of a parent, these count as examples of conceptual reduction. These explications of meanings are just the result of fully specifying the nature of our concepts as they stand. These are very simple examples, but the more complex instances of conceptual reduction in philosophy follow the same general idea as these ones.

Metaphysical reduction on the other hand has to do with facts in the world and how they stand in relation to each other. I take it that the following necessary condition imposes a restriction on the relation of metaphysical reduction:

• (R) If fact F metaphysically reduces to fact F' then (i) fact F holds in virtue of fact F' holding and (ii) the holding of fact F is nothing over and above the holding of fact F'.

As an example, physicalists often say that all mental facts are reducible to physical facts. I take it that this at least means that the mental facts hold in virtue of the physical facts and that they are nothing over and above the physical facts.

Now, (i) and (ii) seem to me to be in tension with each other. In fact, on the most straightforward reading of (ii) their simultaneously holding leads to a contradiction. Hence, we must find some other way to explain (ii), since it does not seem like a primitive relation. This is rather difficult. Let me explain.

By (i), reducibility must be an asymmetrical relation. This means that if F reduces to F' then F' does not reduce to F. For suppose F reduces to F'. Then F holds in virtue of F'. But the 'holding in virtue of' relation is asymmetrical, since otherwise there would be circular chains of ontological dependence. So if F holds in virtue of F', then F' does not hold in virtue of F, and thus by (R), F' is not reducible to F.

The problem is that the most straightforward reading of (ii) is that the holding of fact F is identical with the holding of fact F'. After all, suppose F and F' are not identical and we are dealing with a world of just F and F' (here I'm abbreviating, and I should really be saying the holding of F and the holding of F'). Then there is a perfectly clear sense in which F is something over and above F', viz. there are more things in the world than F! For if F =/= F', then for some x, x =/= F'. So there is something out there in the world which is extra-mentally distinct from F'. That seems to be a legitimate sense in which F is something over and above F'. So if F is not something over and above F' then F = F'.

But of course, if that were the case, then the 'in virtue of' relation here would not be asymmetrical, since if F = F' and F holds in virtue of the holding of F', then by substitution of equals F' holds in virtue of the holding of F. So reducibility would not, in fact, be asymmetrical. And that is a contradiction, since we earlier established it was.

One option is to say that the 'in virtue of' relation is not asymmetrical. But that seems deeply problematic insofar as it doesn't allow us to capture the reducibility we want to pick out. After all, every materialist will accept that all mental facts reduce to physical facts, but no materialist would ever dare say the physical facts reduce to the mental facts! (Personally I find the latter suggestion more plausible than the former, but regardless it is not something the materialist would ever claim.)

Instead, we have to find a sense in which one could say fact F is nothing over and above F' even though F is not identical to F'. And I'm not sure how to explain this. No idea if this works or not, or whether it is at all helpful, but here's a thought: Let us denote by 'a full truthmaker of P' a truthmaker of P which is not a constituent or part of some other truthmaker of P. Let Q be the proposition expressing the holding of F. Maybe we can say F is nothing over and above F' if the set of all full truthmakers of the proposition Q contains only F'. That would make (i) superfluous it seems. Or at least from pretty uncontentious premises (i) would follow as a consequence. This theory is a little weird though, since the question arises as to what, metaphysically speaking, explains why Q would be distinct from the proposition expressing the holding of F'.

With that said, I don't know if that's on the right track. And even if it gets the extension of the relation right it might not even produce a deeper understanding. The point being, I don't myself know how to explain (ii). Like I said though, it doesn't seem like this is a primitive or undefinable relation. I wonder then what we can say about it.

What God Knew and Abraham Didn't

The traditional story of Abraham and Isaac is one of the most perplexing parts of the Bible, at least for philosophers. There seems to be some sort of implicit contradiction in the idea of God commanding someone to sacrifice a human being, especially an innocent boy. Moreover, it seems like if one were commanded to do this one should not do it. Before thinking about this more we should review the story very quickly.

In the recounting of the story in Genesis 22, God wishes to test Abraham and see whether he "fears God." To this effect, God commands Abraham to go and sacrifice his only son, Isaac. On the third day of their journey Abraham takes Isaac up to a mountain to sacrifice him. Before going up, Abraham tells his servants with him, "We shall worship and come back to you." (Genesis 22:5) Abraham then binds Isaac and prepares to sacrifice him. When Abraham grabs his knife to kill Isaac an angel sent from God stops him by telling him not to kill the boy. God speaks through the angel and says that he now knows that Abraham fears him, and because of his actions God will shower blessings upon Abraham and his descendants.

Sometimes opponents of Christianity will say that this verse proves an inconsistency in the Christian conception of God. On the one hand, God is supposed to be a perfect being, and a perfect being, it seems, would never command something intrinsically evil such as sacrificing an innocent person to him. On the other hand, the Bible says he does. Let's give a precise argument which captures the force of this more vaguely formulated one.

The Universe is Contingent (And Therefore Needs an Explanation)

One common fallacy is the fallacy of composition, where one argues from the fact that each part of a thing has a certain feature to the conclusion that the whole thing has that feature. For instance, one could argue that every brick of the house is cube-shaped, therefore the house is cube-shaped. Or one could argue that each part of one's brain is unconscious, therefore the whole brain is unconscious. These inferences are fallacious.

However, I think it is worth noting that not all inferences from properties of parts to properties of the whole are invalid. If each part of a wall is made entirely of stone, then the whole wall is made entirely of stone. Similarly, if each part of the ball is entirely red, then the whole ball is entirely red. And so on.

Contingency seems to be like this, at least in this case. So here's an argument that the universe must be contingent:

Quantifier Variance and the Semantics of Quantifiers

In my previous post I explained the basic idea behind quantifier variance. Now I want to criticize it. In particular, I said I want to point out some problems with the quantifier variantist's simultaneously affirming the following two statements:

(i) the different quantifiers behave the same logically; and

(ii) the different quantifiers have different meanings.

Let's do a little basic semantics. Let's define the truth function τ[ψ]^U,g relative to models U and g for the cases of quantified formulas ψ as follows. The following definitions are true for all models M, all variable assignments s, all variables x, and all formulas φ. If a formula is not assigned to T it is assigned to F:


τ : {<ψ,U,g>|ψ is a formula, U a model, g a var. assign.} → {T,F}

• (τ-∀): τ[∀xφ]^M,s = T ⇔ for all variable assignments s′, if for all variables v, s(v) ≠ s′(v) ⇒ v = x, then τ[φ]^M,s′ = T 
• (τ-∃): τ[∃xφ]^M,s = T ⇔ for some variable assignment s′, for all variables v, s(v) ≠ s′(v) ⇒ v = x, and τ[φ]^M,s′ = T 

Basics of Quantifier Variance

When I say that there are tables is it unambiguous what I'm saying? Quantifier variantists say no. Or at least they would say that in certain contexts it is not. In particular, the sentence is ambiguous when we are engaging in metaphysical debate about the existence of the table, as in the following case.

Consider the debate between what I will call compositionalism and anti-compositionalism. Compositionalism is the thesis that there are composite material objects, while anti-compositionalism is the thesis that there are not. Take the case of a world with just a table and its parts, and suppose we are considering a form of compositionalism which says there are tables. Assume further that there are exactly n atoms which, according to this form of compositionalism, are proper parts of the table. Note that we are using a philosophical definition of 'atom', according to which an atom is a material object which has no proper parts. Anti-compositionalism says there is no table; there are just the n atoms. 

In essence, compositionalism says (A) there are n+1 distinct things (viz. the n atoms, plus the table), while anti-compositionalism says (B) there are n things and there are no more than n things. Note that (A) and (B) can be adequately translated into a quantified language which only contains variables, quantifiers, sentential connectives, and the identity sign with the usual interpretation. For example, (A) would be translated as follows:

∃x₁∃x₂...∃x_n((x₁≠x₂ ∧ ... ∧ x₁≠x_n+1) ∧ (x₂≠x₃ ∧ ... ∧ x₂≠x_n+1) ∧ ... ∧ (x_n≠x_n+1))

Pure Actuality

Many scholastic theologians, most notably Aquinas, make the claim that God is "pure actuality." This is supposed to do a lot of philosophical and theological "work"; it is by showing that there exists a being which is pure actuality that Aquinas is able to deduce many of the divine attributes. However, it is not immediately clear what this even means if one is not familiar with the metaphysical context of medieval philosophy.

A charitable interpreter who has read some medieval philosophy may be able to see how scholastics use this claim and identify certain inferences from this claim as being valid and others not. But it'd be nice if we had a more precise characterization of what it means to say God is 'pure actuality', so that we can see if all that Aquinas says follows actually does follow from this claim. Moreover, once we have a precise characterization of what Aquinas is even asserting, we can begin to more clearly assess the plausibility of the claim itself and whether Aquinas has established it. I propose the following definition:

• x is pure actuality if and only if for all (intrinsic) P, if x is P then x is actually P.

For completeness and wider scope of application, I also propose the following definitions of a thing's being 'composed of' or 'having' actuality and potentiality:

• x is composed of potentiality if and only if for some (intrinsic) P, x is P and x is potentially P
• x is composed of actuality if and only if for some (intrinsic) P, x is P and x is actually P.