Overview

After discussing Leibniz’s ideas about substance, we spent the bulk of our time talking about how he used his doctrine of pre-established harmony to address the problem of mind-body interaction.

Leibniz’s Mill

Leibniz does not find materialist accounts of the mind satisfactory. To make his point, he asks you to imagine walking into a machine, like a mill. We would only find mechanisms that push against one another, he says, and never find perception or thought.

We must confess that the perception, and what depends on it, is inexplicable in terms of mechanical reasons, that is, through shapes and motions. If we imagine that there is a machine whose structure makes it think, sense, and have perceptions, we could conceive it enlarged, keeping the same proportions so that we could enter into it, as one enters into a mill. Assuming that, when inspecting its interior, we only find parts that push one another, and we will never find anything to explain a perception. And so, we should seek perception in the simple substance and not in the composite or in the machine. Furthermore, this is all one can find in the simple substance—that is, perceptions and their changes. It is also in this alone that all the internal actions of simple substances can consist. (Leibniz [1714] 2019, 304R)

As far as I know, Leibniz began a long tradition of thought experiments involving rooms.

Anyway, the main problem with dualism, according to Leibniz, is that it cannot explain interaction between the body and the mind. We have known that for weeks. Malebranche’s occasionalism would solve the problem: God could make it so that a thought could cause the body to move. But Leibniz is not crazy about that. He thinks that it is like saying that a miracle occurs every time something happens.

Instead, Leibniz proposes his own solution: pre-established harmony. Bodies and minds change together, in harmony. Why? God set it up that way.

We must say that God originally created the soul (and any other real unity) in such a way that everything must arise for it from its own depths, through a perfect spontenaity relative to itself, and yet with a perfect conformity relative to external things. And thus, since our internal sensations (meaning those in the soul itself, and not those in the brain or in other subtile parts of the body) are merely phenomena which follow upon external beings, or better, thay are true appearances and like well-ordered dreams, these internal perceptions in the soul itself must arise because of its own original constitution; that is, they must arise through the representative nature (capable of expressing external things as they relate to its organs) given to the soul from creation, which constitutes its individual character. … There will be perfect agreement among all these substance, producing the same effect that would be noticed if they communicated through the transmission of species or qualities, as the common philosophers imagine they do. (Leibniz [1695] 2019, 301)

That was body to mind or perception. It works in the mind to body direction too, that is, action.

In addition, the organized mass, in which the point of view of the soul lies, being expressed more closely by the soul, is in turn ready to act by itself, following the laws of the corporeal machine, at the moment when the soul wills it to act, without disturbing the laws of the other — the spirits and blood then having exactly the motions that they need to respond to the passions and perceptions of the soul. (Leibniz [1695] 2019, 301)

In sum:

It is this mutual relation, regulated in advance in each substance of the universe, which produces what we call their communication, and which alone brings about the union of soul and body. (Leibniz [1695] 2019, 301)

It is not obvious to me that he is being fair to Malebranche. Malebranche thought that God operates through laws. So is his view all that different? Maybe! I present, you decide.