The Abhidharma school of Indian Buddhism represents one of the earliest attempts to form a complete, coherent philosophical system based on the teachings of the Buddha. Abhidharma metaphysics rests on mereological reductionism: the claim that wholes are reducible to their parts. On the Abhidharma view, a composite object like a table is nothing more than its parts arranged table-wise. The “table” is a convenient designator based on our shared interests and social conventions. Crucially, for Abhidharma Buddhists, this also extended to the self. The self, rather than being an enduring substance, is reducible to a bundle of momentary mental states (Carpenter, 2).
Based on this principle of reductionism, Abhidharma went on to develop the Doctrine of Two Truths. A statement is “conventionally true” is if it is based on our commonsense view of the world, and leads to successful practice in daily life. Thus, it is conventionally true that macro objects such as tables and chairs exist. A statement is “ultimately true” if it corresponds to the facts as they are, independent of any human conventions. According to the Abhidharma view, the only statements that can be considered ultimately true are statements about ontological simples: entities that cannot be further broken down into parts. The tendency to think that statements involving composite objects like tables are ultimately true arises when we project our interests and conventions on to the world.
The primary opponents of the Abhidharma Buddhists were philosophers of the Nyāya orthodox tradition, about which I have written before. Nyāya philosophers were unflinching commonsense realists. They held that wholes existed over and above their parts. The word “table” is not merely a convenient designator or a projection of our interests on to the world, it is a real object that cannot be reduced to its parts. Nyāya held that there are simple substances and composite substances. Simple substances are self-existent and eternal. Composite substances depend on simple substances for their existence, but cannot be reduced to them. They possess qualities that are numerically distinct from the qualities of their component parts.
There are some obvious difficulties with the view that wholes exist in addition to their parts, and Abhidharma philosophers were quick to point this out. If the table exists in addition to its parts, it would follow that whenever we look at a table, we are looking at two different entities – the component parts and the (whole) table. How can two different objects share the same location in space? Nyāya philosophers responded by stating that wholes are connected to parts by the relation of inherence. In Nyāya metaphysics, inherence is an ontological primitive, a category that cannot be further analyzed in terms of something else. To put it very crudely, inherence functioned as a kind of metaphysical glue in the Nyāya system. The inherence relation is what connects qualities to substances. The quality redness inheres in a red rose. Similarly, the inherence relation also connects wholes with their parts. In this case, the whole – the table – inheres in its component parts.
At this point in the debate, the standard Abhidharma move was to ask how exactly wholes are related to their parts. Do wholes inhere wholly or partially in their parts? If wholes are real and not reducible to their parts, but nonetheless inhere only partially in their parts, it would mean that there is a further ontological division at play. We now have three different kinds of entities. The parts of the table, the parts of the whole that inhere in the parts of the table, and the whole. Now, what is the relation between the whole and the parts of the whole that inhere in the parts of the table? Does the whole inhere wholly or partially in the second set of parts? If it is the former, then the second set of parts becomes redundant, for the whole could simply inhere wholly in the first set of parts (that is, the parts of the table). If the whole inheres partially in the second set of parts, then we will have to introduce yet another whole-part distinction, and there is an obvious infinite regress looming.
The Nyāya school held that wholes inhere wholly in their component parts. They drew an analogy with universals to make the illustration clear. Just as the universal cowness inheres in every individual cow, the table inheres wholly in every one of its individual parts.
Abhidharma philosophers rasied a second set of difficulties for Nyāya. Consider a piece of cloth woven from different threads. According to the Nyāya view, the cloth is a substance that is not merely reducible to the threads. But now let us suppose I cannot see the whole cloth. Let us suppose most of the cloth is obscured from my view, and I only see a single thread. In this case, we would not say that I have seen the cloth. I am not even aware that there is a cloth – I think there is just a single thread. But if the Nyāya view is correct, then the cloth (the whole) inheres in every single thread, so when I see the thread, I should see the cloth as well. But since I don’t, it follows that the Nyāya view is incorrect.
Now consider a piece cloth woven from both red and black threads. Since the cloth is a separate substance, and since composite substances possess qualities numerically distinct from their component parts, the cloth must have its own color. But is the color of the cloth red or black? Nyāya responded that the color of the cloth is neither red nor black, but a distinct “variegated” color (Siderits, 111). But this only multiplies difficulties. If the cloth is wholly present in its parts, and it possesses its own variegated color, why do I not see the variegated color when I look at its component parts? When I look at the red threads, all I see is red, and when I look at the black threads, all I see is black. I do not see the variegated color in the component parts and yet, if the whole inheres wholly in its parts, I should.
Finally, if the whole is a distinct substance over and above its parts, the weight of the whole must be greater than the sum of its parts. But we do not observe this when we weigh composite substances. This is highly mysterious on the Nyāya view. But these problems are all avoided if we simply accept that wholes are reducible to their parts.
Abhidharma is a broad tradition that encompasses numerous sub-schools. Two of the most prominent ones are Vaibhāṣika and Sautrāntika. While both sub-schools agree that everything is reducible to ontological simples, they disagree on the number and nature of these simples. The Vaibhāṣika school is fairly liberal in its postulation of simples, while Sautrāntika is conservative. Moreover, Vaibhāṣika treats simples as bearers of an intrinsic nature. According to Vaibhāṣika atomists, an earth atom, for instance, is a simple substance that possesses the intrinsic nature of solidity. The Sautrāntika school rejected the concept of “substance” entirely. There are numerous reasons for this (most of them epistemological, that I will cover in a subsequent essay), but roughly, it came down to this: We have no evidence of substances/bearers, only qualities. Further, there is no need to posit substances, because everything that needs to be explained can be explained without them. For Sautrāntika philosophers, an earth atom is not a substance that is the bearer of an intrinsic nature “solidity” – rather, it is simply a particular instance of solidity. Thus, in Sautrāntika metaphysics, there are no substances or inherence relations, there are simply quality-particulars. This position is similar to what contemporary metaphysicians call trope theory.
The term “reductionism” is often cause for confusion when used in relation to Abhidharma Buddhism. It must be emphasized that the kind of reductionism relevant here is mereological reductionism. Abhidharma Buddhists were not reductionists in the sense of believing that consciousness could be reduced to material states of the brain. All Abhidharma schools held that among the different kinds of ontological simples, some were irreducibly mental, as opposed to physical.
Apart from mereological reductionism, the other key aspects of Abhidharma metaphysics are nominalism and atheism. I have covered the Buddhist approach to nominalism in a previous essay, so I will not go over it here. When it comes to atheism, it is important to recognize that Abhidharma Buddhists (like all Buddhists) were only atheistic in a narrow sense. They rejected the existence of an eternal, omnipotent creator of the universe. This did not mean that they were naturalists or that they rejected deities altogether. They believed in many gods, but these gods were not very different from human beings apart from being extraordinarily powerful. Venerating the gods was a means of obtaining temporary benefits in this life or a good rebirth, but the gods could offer no help with the ultimate goal of Buddhist practice: liberation from the cycle of rebirth. The gods themselves, being unenlightened beings, were stuck in the cycle of rebirth. To attain liberation one must seek refuge in the Buddha, the teacher of gods and men.