Some thoughts on the skandha doctrine

According to a long-standing Buddhist tradition, a person consists of five parts, each of which is called a skandha:

1. body (rūpa, "form")
2. sensation (vedanā, sometimes rendered into English as "feeling")
3. perception (saṃjñā)
4. formations (saṃskāra)
5. consciousness (vijñāna)

This list of terms raises five related questions. First, what is consciousness? Second, what is the difference between sensation and perception? Third, what are formations? Fourth, how does perception interact with the other skandhas during the process of perceiving something? Fifth, where do thoughts fit into the list?

What is consciousness?

Within the orthodox Theravada tradition, the first question has a simple answer: consciousness is awareness of an object. More specifically, consciousness is awareness that occurs when an object stimulates a sense-organ.

Buddhism recognizes six sense-organs:

1. The eye detects visible forms.
2. The ear detects sounds.
3. The nose detects smells.
4. The tongue detects tastes.
5. The body detects tactile qualities.
6. The mind detects mental objects. (We will examine what those mental objects are later.)

When an object stimulates a sense-organ, the result is awareness, or consciousness, of the object. There are six kinds of consciousness corresponding to the six sense-organs:

1. eye + visible form → eye-consciousness
2. ear + sound → ear-consciousness
3. nose + smell → smell-consciousness
4. tongue + taste → taste-consciousness
5. body + tactile quality → body-consciousness
6. mind + mental object → mind-consciousness

Sensation and perception

The Sattaṭṭhāna Sutta makes it clear that the skandha called "perception" is more or less what we ordinarily mean by the word "perception": the recognition of a sense-object, such as a sound or a taste. For example, to perceive a thunderclap is to recognize that the sound that one is hearing is a thunderclap. This sutta also makes it clear that the skandha called "sensation" is merely the hedonic quality of an experience: the quality of being pleasant, painful, or neutral.

Therefore, we might summarize the relationship between sensation and perception as follows. When an object stimulates a sense-organ, the result is consciousness of the object. Along with this consciousness come two other mental activities: a hedonic quality⁠—pleasant, painful, or neutral—and recognition of the object. The hedonic quality is sensation, whereas the recognition is perception.

What are formations?

Lists of the skandhas often substitute "volition," "choice," or "intention" (cetanā) for "formations" (saṃskāra). On one hand, therefore, we might think that formations are mental formations in the strictest sense: intentions or decisions put together, or formed, by the mind.

On the other hand, we might be tempted to include mere thoughts (as opposed to the intention or decision to do what one is thinking about) in the category of formations. For one thing, it is unclear where else thoughts could go in the scheme of the five skandhas. More importantly, the developed Theravada abhidhamma tradition identifies numerous kinds of formations, many of which are more like thoughts than like intentions (see, for example, Visuddhimagga 14.133).

So what really belongs in the formations category? In a way, this question is meaningless. Like other Buddhist terms, saṃskāra has been used in different ways by different thinkers. For an analogy, consider the term "consciousness" (vijñāna). Theravadin orthodoxy limits consciousness to the six kinds listed above (eye-consciousness, etc.). In Yogacara philosophy, however, the category of consciousness came to include the subconscious, or "storehouse consciousness" (ālāyavijñāna).

Although there is no "right answer" as to what belongs in the formations category, I suspect that the formations category was originally meant to include only intentions, broadly construed (choices, desires, attachments, plans). This interpretation especially makes sense when we consider that Buddhist tradition associates formations, more than the other skandhas, with karma, the consequences of one's moral and immoral intentions.

The Sattaṭṭhāna Sutta tells us that formations, like sensations and perceptions, result from the stimulation of a sense-organ. Thus, we can say that when an object stimulates a sense-organ, the result is consciousness, or awareness, of the object, accompanied by three things:

• sensation: the hedonic quality of the experience
• perception: recognition of the object
• formation: an intention with regard to the object

The basic perceptual process

We can now explain how the process of perceiving a physical object works according to skandha theory (or at least according to the Sattaṭṭhāna Sutta):

1. An object stimulates a physical sense-organ (body).
2. The stimulation causes awareness (consciousness) of the object.
3. Along with awareness of the object comes pleasure, pain, or neutrality (sensation).
4. Along with awareness of the object comes recognition (perception) of the object.
5. Along with awareness of the object comes an intention (formation) with regard to the object.

Here is an example where the object is the smell of cookies:

1. The smell of cookies stimulates the nose.
2. The stimulation causes awareness of the smell.
3. Along with awareness of the smell comes pleasure because the smell is delicious.
4. Along with awareness of the smell comes recognition that the smell is the smell of cookies.
5. Along with awareness of the smell comes the intention to eat a cookie.

How do thoughts fit in?

It's unclear how thoughts are supposed to fit into this scheme. Consider, for example, the thought that it is raining⁠—not the bare awareness of rain, not the sensation that you get from seeing or feeling rain, not the perception of rain, but rather the thought, the idea, that it is raining. Where does that thought fit into the list of five skandhas? I have a tentative solution that, I think, makes good sense of the model that we have already built.

The first step is to bring the mind into the picture. Recall that the mind is simply one of the sense-organs. While the other sense-organs detect physical objects, the mind detects mental objects. There is no shortage of mental objects already present in the list of skandhas: sensation, perception, and formations are clearly mental.

Given our model of perceiving a physical object, we can build an analogous model of perceiving a mental object:

1. A mental object stimulates the mind.
2. The stimulation causes awareness (consciousness) of the object.
3. Along with awareness of the object comes pleasure, pain, or neutrality (sensation).
4. Along with awareness of the object comes recognition (perception) of the object.
5. Along with awareness of the object comes an intention (formation) with regard to the object.

Here is an example in which the mental object is the intention to eat a cookie:

1. The intention to eat a cookie stimulates the mind.
2. The stimulation causes awareness of the intention.
3. Along with awareness of the intention comes pain because one is trying not to eat cookies.
4. Along with awareness of the intention comes the recognition that one intends to eat a cookie.
5. Along with awareness of the intention comes the intention to refrain from eating a cookie.

Here is an example in which the mental object is the recognition that a smell is the smell of cookies:

1. The recognition that a smell is the smell of cookies stimulates the mind.
2. The stimulation causes awareness of the recognition.
3. Along with awareness of the recognition comes pleasure because one enjoys identifying the smells of various baked goods.
4. Along with awareness of the recognition comes the recognition that one recognizes that the smell is the smell of cookies.
5. Along with awareness of the recognition comes the intention to see whether one can recognize the specific kind of cookie from the smell. 

As you can see, the mental objects⁠—sensations, perceptions, and formations⁠—that result from one perceptual process can stimulate the mind, producing an endlessly proliferating series of perceptual processes, each taking as its object one of the outputs from another perceptual process. 

I know of no Buddhist text that says so explicitly, but given our model of the perceptual process, I think it makes most sense to say that what we normally call "thinking" extends throughout this entire complicated series of perceptual processes. On this model (with which I'm not sure I agree), we might be able to equate "thoughts" with certain skandhas (for example, perceptions and formations), but thinking is not any particular skandha. Instead, mental objects produced by our minds stimulate our minds, producing yet further mental processes in the complex and, in principle, endless process that we call thinking.