157. Leibniz’s Mill and the Nature of Perception

In this post I would like to explore, with the help of some insights from G.W. Leibniz (1646-1716), a few interesting yet controversial points about perception and how these points suggest perception is not something that occurs in the brain.

Perceptions are Unities, the Brain is Complex

In section 14 of his work Monadology, Leibniz claims that “The passing state that involves and represents a multitude in the unity, or in the simple substance, is nothing but what is called perception.” For Leibniz, perceptions are unities. Look around you right now: do you not experience a multitude of diverse sensations unified into one perceptual experience of your environment? Is it not the case that your experience, rather than being perceived as some diverse aggregate, is experienced as unified? It certainly seems so. But this shows, according to Leibniz, that perception can’t be mechanically explained. Consider the thought experiment he presents in section 17:

“One is obliged to admit that perception and what depends upon it is inexplicable on mechanical principles, that is, by figures and motions.  In imagining that there is a machine whose construction would enable it to think, to sense, and have perception, one could conceive it enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill. Supposing this, one should, when visiting within it, find only parts pushing one another, and never anything by which to explain a perception. Thus it is in the simple substance, and not in the composite or the machine, that one must look for perception.  Moreover, there is nothing besides this—besides perceptions and their changes—that one could possibly find in a simple substance. It is also in this alone that all the internal actions of simple substance can consist. ”

This wonderful thought experiment, often referred to as Leibniz’s Mill, helps us see that unified perceptions could never be fully explained by brain analysis however complex. Even if we enlarged the brain and examined every part and mechanism of it we would never find the means to fully account for perceptions. This is because the brain is an object made of parts and perceptions are not made of parts. Thus perceptions are not reducible to brain states: something more would be required to explain them. But what?

Perceptions are Private, the Brain is Public

Well, if everything in the physical world is comprised of aggregates, of complex things and events with parts, then, as Leibniz says, it is to the “internal actions” of a “simple substance” that we should look to find perceptions. For Leibniz, this simple substance is an immaterial mind or, as he refers to it, a “monad.” According to Leibniz, monads are needed since only a true unity can unify. As he writes:

“A thing which can be divided into several (already actually existing) is an aggregate of several, and…is not one except mentally, and has no reality but what is borrowed from its constituents. Hence I inferred that there must be in things indivisible unities, because otherwise there will be in things no unity, and no reality not borrowed.” (See Nicholas Rescher’s G.W. Leibniz’s Monadology: An Edition for Students, p. 50).

Moreover, according to Leibniz mechanical action is simply a matter of cause and effect or efficient causality without regard to purpose. But perception is about final causality or the type of action that is goal-oriented or teleological such as pursuing good and avoiding evil: “The perceptions in the monad arise from each other according to the laws of the appetites or of the final causes of good and evil…” (Rescher,  p. 85).

Thus perception would not be taking place in the material brain since, on the one hand, we need simple unities to unify in accordance with goals and, on the other hand, the brain is complex and can be understood with non-teleological mechanical principles alone. Rather, perception would be an action “internal” to monads.

Now these points suggest that we cannot access other people’s perceptions and experience them from their perspective. When we experience things from a third person perspective we experience them outside of us in a public space where others can, in principle, experience them as well. For example, a weapon which is presented as evidence of a murder is something that can be observed from a third person perspective: it is an object in the environment to which everyone has, in principle, access. It is empirically verifiable, that is, it can be shown to exist through the five senses and can therefore serve as evidence in constructing an argument in a court of law.

But our perceptions of such observable objects do not appear to be accessible from this third person perspective. Perceptions appear to be from a first person perspective, a perspective from the inside, and cannot be known by anyone else in that way. For example, I may come to know, based on a complete understanding of your brain states, that you are in pain. But I can’t know what it is like, from your perspective, to be perceiving your pain. Thus for Leibniz perceptions, insofar as they occur in immaterial simple substances, are essentially private whereas material aggregates like the brain can always, in principle, be made public.

Thus Leibniz’s Mill isn’t just making the point that perceptions cannot be mechanically explained by analyzing aggregates of material things. It can also be taken to mean that perceptions will never be found in the brain no matter how hard we look. As true unities, purposeful perceptions must be internal to a private, immaterial substance that is truly a purposeful unit. As Leibniz famously said in section 7 of the Monadology, “Monads just have no windows through which something can enter into or depart from them”.

Application of the Identity of Indiscernibles

One of Leibniz’s most famous principles can be usefully applied to the foregoing. Here is the principle:

  • The Identity of Indiscernibles: Two things are identical if, and only if, they simultaneously share exactly the same properties.

Once we understand this principle we see that if mind and matter are the same then they should have the same properties. Thus discovering a property of mind—even one—that is not shared by matter would establish that mental states and brain states are radically different in kind.

Leibniz’s thought experiment attempts to show us that the mind has two properties the brain doesn’t: it has purposeful and unified perceptions that cannot, in principle, be explained through an analysis of complex matter using non-purposeful mechanical principles. Moreover, his claim that perceptions can only take place in a simple, immaterial substance implied that the mind has another property the brain doesn’t: it has perceptions that can only be accessed from a first person perspective. If these points are convincing then, in accordance with the Identity of Indiscernibles, the mind is not a material aggregate and some form of dualism or idealism is established.


These insights are certainly sensible. But they can be challenged. Regarding the first point about the unified nature of perception, we could ask: are perceptions really unities? Leibniz defines perception that way and then moves to his thought experiment for clarification. But what if we don’t accept his definition? To be sure, perceptions seem to be unified. And they certainly seem to be intentional and purposeful. But could it be that they are actually complex and can be explained with non-purposeful mechanical brain analysis alone? And isn’t it plausible to argue, especially in light of everything we know about mental illness and brain damage, that perception depends completely on a set of brain functions? Moreover, we could challenge Leibniz’s assumption that all matter is infinitely divisible, an assumption that, along with his definition of perception, leads him to locate perceptions in monads.

Regarding the second point, it can be argued that from the fact that we all have a subjective, inner, private realm, it doesn’t follow there is a separate simple substance that has such private thoughts and feelings. Perhaps perceptions are in the brain, or at least in its surrounding space like a field, but we simply don’t, and perhaps never will, have third-person access to them. After all, we don’t have direct access to many sub-atomic particles either. Does it follow that such particles are immaterial or reside in an immaterial mind? It doesn’t seem so. And perhaps we will, given enough time and technological innovation, come to see mental thoughts as we can see planets, trees, and amoebas.

Lastly, there are a set of problems that arise if the above arguments work: where do immaterial minds come from? How does one immaterial mind pair up with one particular body? How do the unified minds mentally and purposefully unify aggregates so that bodies seem to have the unity they do? How do immaterial minds interact with material bodies at all? How can we identify such minds over time if we can’t access them directly with our senses? Do we really want to accept a theory so inconsistent with the prevailing naturalism and materialism of the sciences?

These are just some of the questions that would have to be addressed if the above account is to be completely persuasive.

For my post that explores Leibniz’s critique of atoms in relation to a few of his principles, go here.

For my post that explores Leibniz’s view of the soul and its immortality, go here.

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