Leipzig 1646 – Hanover 1716

Gottfried Wilhelm Leibniz

Imagine a machine that thinks, enlarged until you can walk inside it like a mill. You would see gears and levers pushing one another, and nothing that explains a perception. So perception cannot be mechanical.

The view

Leibniz was a mathematician, logician, diplomat and engineer. He invented the calculus independently of Newton, and built one of the first calculating machines that could multiply. Few people of his age knew more about what machines can do. That is what gives his argument against mechanical minds its force.

His metaphysics is strange. The ultimate constituents of reality, he held, are not atoms but monads: simple, indivisible, non-physical units whose whole activity consists of perception and appetite, the drive from one perception to the next. Bodies are aggregates of monads, and a mind is a dominant monad, a single unified perceiver. Most perceptions are tiny and unconscious, the "little perceptions" that add up, like the sound of each wave adding up to the roar of the sea. Only some rise to awareness.

The argument

The mill appears in section 17 of the Monadology (1714). Our translation:

We would find inside only parts pushing one another, and never anything by which to explain a perception. Leibniz, Monadology §17 (1714)

Suppose, he says, there were a machine whose structure made it think, sense and perceive. We could imagine it enlarged in proportion, so that we could go inside as into a mill. Inspecting it, we would find only pieces acting on one another, never a perception. Perception must therefore be sought in a simple substance, not in a composite or a machine. Step inside the machine below.

The argument is the ancestor of many later ones: Nagel's bat, Jackson's Mary, Chalmers's zombies. All of them say, in one form or another, that you can know every mechanical fact and still not have found the experience. That is why the gap between mechanism and perception is sometimes called "Leibniz's gap".

Where he meets Dennett

On unconscious mental life, surprisingly closely. Leibniz's "little perceptions", too faint to notice individually but shaping thought and action, are an early version of the unconscious representations Dennett called "as good as gospel" in cognitive science. Leibniz also saw that consciousness comes in degrees and depends on how perceptions are combined and attended to, an idea not far from Dennett's "fame in the brain".

Where they part

Dennett thought the mill was a lesson in failure of imagination. Of course you see only parts pushing parts, he replied, if you walk through the brain at the scale of its parts. Walk through a computer and you see only electrons; you will not find the word processor, or the game of chess, either. The perception is not a part. It is a pattern in the parts' activity, visible only at the right level of description, from the right stance. Press the second button above: the same gears, seen as a pattern of activity, carry the content "red rose" without any gear being red or being a perception.

This is the same move Dennett makes against stage-magic reasoning. That you cannot see how the parts produce the effect is a fact about you, not proof that the parts cannot produce it.

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Further reading

• G. W. Leibniz, Philosophical Essays, trans. Roger Ariew and Daniel Garber (Hackett, 1989), which includes the Monadology.
• Maria Rosa Antognazza, Leibniz: A Very Short Introduction (Oxford, 2016).
• Daniel C. Dennett, Consciousness Explained (1991), which takes on the mill directly.
• Leibniz's philosophy of mind, Stanford Encyclopedia of Philosophy.