Leibniz's mill argument

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This is the 48th post in my Theories of Knowledge and Reality series. The last post looked at some arguments roughly based on Rene Descartes' arguments for dualism. This post considers a very different sort of argument for dualism from Gottfried Leibniz.

Leibniz asks us to imagine walking around inside a large mill or factory. If we are merely physical, then a tiny person walking around inside your brain would be seeing the same sorts of things you see when walking around the mill. There are physical things going on, things we understand through science. We know exactly what those things involve, and they don't involve thinking. Why, then, should we think a physical organ like a brain involves thinking? So dualism must be true. The main intuition behind this is that physical processes don't seem to explain how thinking can come about. It seems too mysterious for a physical brain to explain.


If the simplest theory explaining the evidence is probably the best, then a theory without all that extra mind stuff is better than one without it, given that they both assume the physical world exists. Leibniz thinks he avoids this, since he claims materialism doesn't explain thinking. He says you need to go beyond the physical world to explain thinking. If so, then dualism is necessary to explain something. The principle of simplicity applies only if you've got two theories that equally explain the evidence, but if dualism explains it better, then go with that.

Materialists reply that he hasn't explained anything. He's said physical things don't explain thinking, but he hasn't said how immaterial minds do any better. What is this immaterial mind thing supposed to be, and how does it explain thinking any better than physical processes? The physical processes may lead to thinking. It's sort of mysterious how they'd do that, but that doesn't mean they don't. Dualism hasn't added anything like an explanation, just the claim that something else is needed. So has the dualist really explained anything? If not, the materialist says, then the simplicity principle tells us to be materialists. Materialism hasn't explained all the evidence, but dualism doesn't do any better, so they're on equal footing in terms of explanations. Then we go with the one without the extra souls, and we hold to the materialist view.


(Keep in mind that the story gets complicated if you include George Berkeley's idealist view, which denies the existence of anything beyond our minds. According to his view, the world is also simpler than dualism, since only minds exist and not external physical objects. In terms of simplicity, this view is as good as physicalism/materialism.)objects. In terms of simplicity, this view is as good as physicalism/materialism.)


So far it looks as if the traditional arguments for dualism from such noted figures as Descartes and Leibniz are good at expressing intuitions that dualists have but not as helpful in offering reasons for materialists to abandon materialism.In the next post, I'll start looking at one of the strongest arguments against dualism to see if the arguments for materialism are any better.


Good post. My first philosophy paper was on this subject :-).

Actually, dualism does solve the problem as long as the soul is indivisible and "consciousness" is a property of the soul, in much the same way that polar charge is an indivisible property of the simplest particle.

Of course, if there were a simple "consciousness particle" materialism would also answer Leibniz's mill too. But science seems to indicate that no such particle exists (look at Thomas Huxley's frog vivisection proof that no part of the body contains consciousness).

So how can dualism work? The soul and body must interact somehow, and Thomas Huxley's argument shows it's not likely a mechanical interaction. Well there is a Heisenberg gap at the atomic level. If dualism is true, it likely acts at that level. If some of the machinery above that level is faulty, there's nothing much the soul can do. It's sort of like a man trapped in a broken car. This is precisely what's observed in cases of brain damage.

Check out Alvin Plantinga's paper "Against Materialism", in which one approach he takes is to develop Leibniz's argument against the notion that a material thing can think. Here is my layman's summary of this part of the argument:

(1) humans have beliefs;
(2) beliefs have content, that is, semantic properties;
(3) on a materialist account, beliefs and other propositional attitudes must be equivalent to brain states;
(4) we can see that brain states cannot have semantic properties;
(5) therefore, beliefs cannot be reduced to brain states;
(6) therefore, materialism cannot account for humans having beliefs and other propositional attitudes in a non-eliminative sense.

Plantinga holds that the immaterial human self is an immaterial simple.

A lot of materialists aren't going to accept the third premise. Even a reductive materialist isn't necessarily going to think brain states are the things mental states reduce to. But non-reductive materialists aren't going to identify mental states as any particular physical state, even if they supervene on them.

Of course the reductivist identity theorist, who thinks a brain state is identical with a physical state, will also deny premise 4. Brain states do have semantic properties but not in any way that you can see when thinking about them under the description "brain states". You have to think of them under their equally legitimate description "beliefs" to see that they do have semantic properties.

The arguments for materialism are often just as bad as this, but enough has been written to show that such arguments do not move materialists when they can just deny a premise and fit their view to say the thing you might have thought no one would say.

I can't help but think this post misconstrues Leibniz's point a bit. As Leibniz himself writes:

"Moreover, everyone must admit that perception, and everything that depends on it, is inexplicable by mechanical principles, by shapes and motions, that is. Imagine there were a machine which by its structure produced thought, feeling, and perception; we can irelative proportions, to the point where we could go inside it, as we would go into a mill. But if that were so, when we went in we would find nothing but pieces which push one against another, and never anything to account for a perception. Therefore, we must look for it in the simple substance, and not in the composite, or in a machine. And that is all we can find within a simple substance, namely perceptions and their changes; and that is all that the internal actions of simple substances can consist in."
(Monadoloy 17)

There are a couple things to note here: First, the force of the argument is against the idea that a thinking subject can be composite. Leibniz is saying, I believe, that a unified consciousness cannot be distributed across so many mechanically operating parts. But (Leibniz elsewhere argues) all material things a composite, so consciousness could not be material. Only a mind could conceivably be simple. The upshot is that perceptions (and thoughts and feelings, for that matter) are not to be sought in a plurality of physical parts pushing and pulling one another, but in a simple substance, an immaterial soul. Leibniz is less worried about how the mind functions, but what's to say what it is.

But isn't the stuff about being composite only in the conclusion? Perhaps he could have put together a more explicit argument along such lines (and then it would be similar to Descartes' argument from divisibility, which does assume things materialists will deny). But here he seems to infer from the impossibility of its being a machine to the requirement of being simple, not the other way around.

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