In an earlier post, I went over an argument for the existence of God that was formulated by philosophers in the Nyāya tradition. Here my aim is to provide a brief summary of some Nyāya arguments for substance dualism, the view that mental and physical substances are radically distinct.
The categories of substance and quality were fundamental to Nyāya metaphysics. A substance is the concrete substratum in which qualities inhere. An apple, for instance, is a substance, and redness is a quality that inheres in it. Substances can be complex and made up of parts (like an apple) or simple and indivisible (like an atom).
Nyāya held that in addition to physical substances, there are non-physical ones. Our individual soul – that is, our Self – is a non-physical substance. Like atoms, individual souls are simple and indivisible, and hence eternal (since destroying an object is the same as breaking it up into its constituent parts, and simple substances do not have any constituent parts). Consciousness, and different conscious states like desires and memories, are qualities that inhere in the substantial Self.
The primary philosophical adversaries of Nyāya belonged to two different camps. The first was Cārvāka, which claimed that only physical substances exist, that the mind does not exist apart from the body, and that the self is reducible to the totality of the body and all its functions. The other was Buddhism, which rejects physicalism but denies the existence of the substantial Self. Buddhism replaces the idea of the Self with a stream of momentary causally connected mental states. Nyāya was engaged in a protracted series of debates with both Cārvāka and Buddhism. Versions of the arguments I summarize in this essay were developed and defended by Nyāya thinkers such as Vātsyāyana (5th century), Uddyotakara (7th century) and Udayana (10th century), among others.
Nyāya came up with a number of arguments against physicalism. The one I focus on here has interesting similarities to arguments found in contemporary debates within the philosophy of mind. It can be stated like this¹:
(P1) All bodily qualities are either externally perceptible or imperceptible.
(P2) No phenomenal qualities are externally perceptible or imperceptible.
(C) Therefore, no phenomenal qualities are bodily qualities.
The argument is deductively valid, so let us examine the premises. As the term suggests, externally perceptible bodily qualities are features of the body that can be directly perceived by external agents. Color is an example of an externally perceptible quality. Everyone who can see me can see that the color of my body is brown. An imperceptible quality is a feature of the body that cannot be directly perceived, but can be inferred through observation and analysis. Weight was a common example used in Nyāya texts. You cannot directly perceive my weight, but if I stand on a weighing scale, you can know my weight by looking at the number displayed by the scale. P1 states that all physical qualities are exhausted by these two categories.
Let us movie on to P2. Phenomenal qualities are the features of conscious experience: the subjective, first person what-it-is-likeness to have an experience. The experience of color, pleasure, pain, desire, and memory are all examples of phenomenal qualities. P2 draws on the intuition that phenomenal qualities are essentially private.
To say that phenomenal qualities are not externally perceptible is to say that I cannot immediately know what it is like for you to have an experience. I have direct access to externally perceptible qualities of your body like color, but I don’t have direct access to your phenomenal qualities. I may be able to infer based on your behavior that you are in pain, but I don’t experience your pain in the immediate, first person manner that you do. The contemporary American philosopher Thomas Nagel made a similar point in his classic paper What Is it Like to Be a Bat? We may be able to observe how bats behave, and how their organs, brain and nervous system work, but we can’t know what it feels like, from the inside, to be a bat. Only a bat knows what it is like to be a bat.
If phenomenal qualities aren’t externally perceptible, perhaps they are imperceptible qualities like weight. But this is extremely implausible. Phenomenal qualities are not externally perceptible, but they’re clearly internally perceptible. The whole point is that I have direct perceptive access to phenomenal qualities – my conscious experiences are given to me in a basic and immediate fashion. Even if I don’t know that my experiences are veridical, I always know what the features of my own experience are. Thus, phenomenal qualities are not imperceptible.
Since phenomenal qualities are neither externally perceptible nor imperceptible, they are not physical qualities. If physicalism is the thesis that only physical substances and their qualities exist, and the above argument is sound, we must conclude that physicalism is false.
Against No-Self Theory
The above argument by itself does not get us to the kind of substance dualism that Nyāya favored. Buddhists, after all, are anti-physicalists, but they do not believe that the Self is an enduring substance that persists through time. Instead, Buddhists view a person as nothing more than a series of sequential causally connected momentary mental states. The 18th century Scottish philosopher David Hume, and more recently, the British philosopher Derek Parfit, came to roughly the same conclusion.
Again, the Nyāya canon has several arguments against the Buddhist no-Self theory, but I will touch on just two of them here. The first of these is that the Self is necessary to explain the first person experience of recollection or recognition. The intuition here is something like this: If I notice a tree and recognize that it is the same tree I saw a few days ago, there has to be a subject that was present both during the first experience and the second one for recollection to occur. Similarly, if the desire to eat a banana arises in my mind at t2 because I remember that I previously enjoyed eating a banana at t1, there has to be a subject that existed during the initial experience that occurred at t1, and persisted through time until the recollection at t2. Without the Self – a substance that endures through these different points in time – the experience of memory is a mystery.
The Buddhist response was that causal connections between momentary mental states could explain the phenomenon of memory. If the mental state at t1 is causally connected to the mental state at t2, that’s all that’s needed for the mental state at t2 to recall the experience at t1. The Nyāya rejoinder was that causal connections were not sufficient to account for how a mental event can be experienced as a memory. When I recognize a tree I saw few days ago, it isn’t just that an image of the previously perceived tree pops into my mind. Rather, my experience is of the form: “This tree that I see now is the same tree I saw yesterday.” In other words, my present experience after seeing the tree involves my recognition of the previous experience as belonging to myself. Similarly, my current desire to eat a banana is based on my recognition of the previous enjoyable experience of eating a banana as belonging to myself. One person does not experience the memory of another, and in much the same way, one mental state cannot remember the content of another. So a single entity that persists through time must exist.
The second argument for the Self takes for granted what we might call the unity of perception. Our perceptions aren’t a chaotic disjointed bundle despite the fact that they arise through different sense organs. There’s a certain unity and coherence to them. In particular, Nyāya philosophers drew attention to mental events that are characterized by cross-modal recognition. An example would be: “The table that I see now is the same table I am touching.” We have experiences that arise through different channels (in this case, my eye and my hand), but there must be something that ties these experiences together and synthesizes them to give rise to a unified cognitive event. In other words, the Buddhist no-Self theory might be able to explain the independent experiences of sight and touch, but for the object of both experiences to be recognized as one and the same, there must something else to which both experiences belong, and which integrates the experiences to give rise to the unified perception of the object. Again, it seems we must admit the existence of the Self.
Needless to say, all these arguments were (and remain) controversial. The debates between Buddhist and Nyāya philosophers got extremely complex over time. They involved increasingly fine-grained analyses of the phenomenology of recollection/recognition, and increasingly technical discussions on the metaphysics of causation. Similar debates took place between other orthodox Indian schools of thought that believed in the Self (Mīmāṃsā, Vedānta, etc.) and their Buddhist no-Self rivals. A good place to start for further reading on this subject would be the collection of essays in Hindu and Buddhist Ideas in Dialogue: Self and No-Self.
 The argument I’ve presented here is based on Kisor Kumar Chakrabarti’s formulation in Classical Indian Philosophy of Mind: The Nyāya Dualist Tradition.