Descartes' project lead him to a problem that he could not overcome: the problem of the interaction between mind and matter. How it is that the mind can influence events? How does the spirit push around material objects? He brought us to the brink of the problem, but then failed to explain the interaction of mind and body or spirit and matter. Gottfried Wilhelm Leibniz (1646-1716) said, "Monsieur Descartes seems to have given up the game so far as we can see." Descartes dropped the ball, but the ball kept rolling and was picked up by other great thinkers. One of those thinkers was the German rationalist Leibniz who put forward an "impressive interlocking metaphysical system."
Leibniz conceived of reality as consisting of God and non-composite, immaterial, soul-like entities called "monads." Leibniz believed that things seem to cause other things because God pre-ordained harmony between all things in the universe.
He was scornful of Descartes' failure to argue his mind-body dualism to its logical end, but he proposed his own version of the mind-body dualism. In his writings, Leibniz talks a good deal about machines such as watches and clocks. In Section 17 of the Monadology he presents an argument about the relationship between mentality and machines. Leibniz’s “mill argument” is as follows:
Moreover, we must confess that perception, and what depends upon it, is inexplicable in terms of mechanical reasons, that is through shapes, size 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 a mill. Assuming that, when inspecting its interior, we will find only parts that push one another, and we will never find anything to explain a perception. And so, one should seek perception in the simple substance and not in the composite or in the machine. (GP VI, 609/AG 215)
Imagine walking around inside a large mill or factory. You are able to see the mechanics and observe the processes, but you do not find a "brain" in the factory to explain how the factory thinks and acquires knowledge. Things operate according to their inventor's design. They run and produce, as is true also of the human, so why do we assume that the human brain involves thinking? Mechanical processes do not explain thinking, therefore, external operations and mental operations are two distinct things. This is Leibniz's version of dualism.
Leibniz’s Critique of Descartes
Leibniz observed that if life is a dream (as Nietzsche asserted), it must still have structure. He applied this idea to the relationship of objects. According to Leibniz, every entity whether mental or physical is independent and constitutes a “monad.” Each monad is fixed or determined in its properties according to its essence or nature. Whatever form an entity takes or whatever happens to an entity is entirely determined by its essential characteristics and not from the influence of any other entity (a version of essentialism).
Leibniz overcame Descartes’ dualism by claiming that there is no connection between mind and body, but a harmonization of all (Monism) on the level of kinetic energy. Leibniz believed that the world is not as Descartes claims. It has structure, but what we see is merely the visible outcome of infinitely numerous spiritual things that are not complex and therefore not divisible. Today we might speak of “monads” as the most fundamental units of energy.
Leibniz disagreed with Descartes that God is the mechanism that makes connection of mind-body possible. That would be to cast God as an extension of the mind and would suggest that God can be reduced to constituent parts or analyzed into simpler elements. For Leibniz, the ultimate constituents of the world must be non-material and therefore not divisible. These cannot occupy space and cannot be said to have extension. In this view, God is not a mechanism that connects things, but instead the mind that harmonizes all things. Here Leibniz approaches Baruch Spinoza’s thought. In fact, the two rationalists had spoken face to face. Spinoza conceived of God as “a being absolutely infinite, i.e., a substance consisting of an infinity of attributes, of which each one expresses an eternal and infinite essence.” Leibniz would have agreed with Spinoza’s statement that “In nature there is nothing contingent, but all things have been determined from the necessity of the divine nature to exist and produce an effect in a certain way.”
Leibniz believed that material and non-material entities do not influence one another, but there is a pre-established harmony between all entities. Consider the example of two clocks that keep perfect time and are exactly synchronized. One has a bell that rings when the hour is struck. The other has no bell; when the one clock points to the hour, the other clock rings precisely at the moment. Descartes would argue that the clocks have some connection and the mechanism that connects is God. Leibniz would argue that although the clocks have no relation to each other, they keep perfect time because from the outset they were synchronized by God.
Just as Descartes begins with skepticism to overthrow skepticism so Leibniz begins with Cartesian dualism to overthrown it with his Monism.
Related Reading: Theories of Knowledge (Part 1); Leibniz on Descartes' Principles