The block quotes are from Ms. O;'Brien:
What are the skandhas? Here is a basic guide. (The non-English names given for the skandhas are in Sanskrit unless otherwise noted.)
The First Skandha: Form (Rupa)
Rupa is form or matter; something material that can be sensed. In early Buddhist literature, rupa includes the Four Great Elements (solidity, fluidity, heat, and motion) and their derivatives. These derivatives are the first five faculties listed above (eye, ear, nose, tongue, body) and the first five corresponding objects (visible form, sound, odor, taste, tangible things).
Another way to understand rupa is to think of it as something that resists the probing of the senses. For example, an object has form if it blocks your vision -- you can't see what's on the other side of it -- or if it blocks your hand from occupying its space.
In one sense, the Skandha of Form is the trickiest to think about. Why? Because it is the suggestion of what is beyond thinking by using objectifying language. In other words it is the psychic view of "the physical." As Carl Jung pointed out, once we take the POV of psychology, we have to stand within the field of the psyche and can no longer pretend that we are standing within the field of the physis or physical world.. In other word what we call "form", "matter", "material", "things" are all categories or differentiations of mind. This is the view of the Lankavatara and the other Mahayana sutras that teach the unity of mind. Once the unity of mind is recognized there is no way to "step outside" of mind. Therefore there is no "form" outside of the differentiation of mind, only our attempts to develop a consensual reality. This why the first skandha is all important but so often overlooked in discussion of the skandhas. In Zen, when phrases like "the bottom of the bucket opens" or "dropping mind and body", it is the letting go of the materialist view of the first skandha that is being referred to by the "bottom breaking" or "dropping body."
Each skandha has one or more core polarities associated with it, indeed we can say it is the field of that polarization that makes the skandhas identifiable as a numbered skandha. With the first skandha, it is the polarization of "inside and outside", "subjective and objective", that is primary.
The Second Skandha: Sensation (Vedana)
Vedana is physical or mental sensation that we experience through contact of the six faculties with the external world. In other words, it is the sensation experienced through the contact of eye with visible form, ear with sound, nose with odor, tongue with taste, body with tangible things, mind (manas) with ideas or thoughts.
It is particularly important to understand that manas -- mind -- in the skandhas is a sense organ or faculty, just like an eye or an ear. We tend to think that mind is something like a spirit or soul, but that concept is very out of place in Buddhism.
Because vedana is the experience of pleasure or pain, it conditions craving, either to acquire something pleasurable or avoid something painful.
The Third Skandha: Perception (Samjna, or in Pali, Sanna)
Samjna is the faculty that recognizes. Most of what we call thinking fits into the aggregate of samjna.
The word "samjna" means "knowledge that puts together." It is the capacity to conceptualize and recognize things by associating them with other things. For example, we recognize shoes as shoes because we associate them with our previous experience with shoes.
When we see something for the first time, we invariably flip through our mental index cards to find categories we can associate with the new object. It's a "some kind of tool with a red handle," for example, putting the new thing in the categories "tool" and "red." Or, we might associate an object with its context -- we recognize an apparatus as an exercise machine because we see it at the gym.
I very much disagree with Ms. O'Brien's statement that most of what we call thinking fits into the samjna skandha. Thinking is much more complex and in fact we don't even have conscious thinking until the fifth skandha so most of what we "call" thinking is conscious thinking, so is not often even considered at these unconscious levels of the first four skandhas.
This skandha is of course as she points out integral to the formation of thinking because it is the beginning of the putting together of the differentiations of mind that becomes thinking. It is the third skandha that begins to relate all the items previously labeled according to the four boxes of the first two skandhas, and by using these four-colored building blocks the third skandha puts them together into the nearly infinitely varied patterns (dharmas) of our consciousness code like the four amino acids of DNA are put together to form the mind boggling number of combinations in the genetic code.
It is this conceptualization of associations that we call "perception" in that we are now beginning to be able recognize patterns and call them things (dharmas). Here the multitude of polarities abound and there is no primary polarity as associated with the first, second, and fourth skandhas. That is, the entire world is polarized with such polarities as "soft and hard", "smooth and rough", "male and female", etc.
The Fourth Skandha: Mental Formation (Samskara, or in Pali, Sankhara)
All volitional actions, good and bad, are included in the aggregate of mental formations. How are actions "mental" formations? Remember the first lines of the dhammapada (Acharya Buddharakkhita translation)--
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with a pure mind a person speaks or acts happiness follows him like his never-departing shadow.
The aggregate of mental formations is associated with karma, because volitional acts create karma. Samskara also contains latent karma that conditions our attitudes and predilections. Biases and prejudices belong to this skandha, as do interests and attractions.
This is the most psychologically challenging of the skandhas to understand. In the context of Jungian archetypal psychology, the fourth skandha is all the mental formations that at one end are the individual complexes and at the other end are the archetypes of the collective unconscious. The "ego" or "self" is only one of the complexes, though it is the individual pole of the primary axis of the mental formations that at the other end is the collective pole that goes by the names of "Self", "God", "Atman" etc. The two ends of this primary axis of mental formations are the "I thou" relationship. However, the mental formations are not a completely connected structure, but more like a solar system or galaxy in which many formations are whirling around in a gravitational relationship but have their own sense or appearance of autonomy in various circumstances.
It is the appearance of autonomy within the central mental formation of the "ego complex" that is the basis for our sense of personal independence and identity. It is this sense of personal independence and identity that is the basis for our self-mage of responsibility and intention. It is this sense of responsibility and intention that is the basis for our karma. That is why the fourth skandha is sometimes identified as "volition."
Ms. O'Brien's saying "All volitional actions, good and bad..." points to another important factor. It is with the fourth skandha that the primary polarity of "good and bad" is developed. At the level of the second skandha there is "pleasure and pain" that acts as the seed of the polarity of "good and bad", but it is not until the activity of the fourth skandha that second skandha polarity becomes developed in the mental formations and "good and bad" are materialized. This is why pleasure is not the sole determining factor of what is "good" though it is a significant factor. The ability to substitute an ideal as a higher good than pleasure is a function of the mental formations as the ego complex interacts with other complexes that create identities such as tribal or national identity in which we would sacrifice our personal pleasure for a greater good. In the religious context it is those factors that become associated with the "god pole" of the central axis that become identified with "the greatest good." For those who do not identify the collective pole of the central axis with a personality image such as a father god or mother goddess, other images may act as substitutes such as "the people", "the nation", "money," "family" etc.
In Zen, getting free from the control of the polarized fields of the fourth skandha's mental formations is what is referred to as "not thinking good and evil."
The Fifth Skandha: Consciousness (Vijnana, or in Pali, Vinnana)
Vijnana is a reaction that has one of the six faculties as its basis and one of the six corresponding phenomena as its object. For example, aural consciousness -- hearing -- has the ear as its basis and a sound as its object. Mental consciousness has the mind (manas) as its basis and an idea or thought as its object.
It is important to understand that consciousness depends on the other skandhas and does not exist independently from them. It is an awareness but not a recognition, as recognition is a function of the third skandha. This awareness is not sensation, which is the second skandha. For most of us, this is a different way to think about "consciousness."
It is also important to remember that vijnana is not "special" or "above" the other skandhas. It is not the "self." It is the action and interaction of all five skandhas that create the illusion of a self.
Having an appreciation of the fifth skandha of consciousness and how it functions within the scheme of analysis known as the five skandhas is essential to understanding the wisdom pointed to by the analytical framework.
Ms. O'Brien's caveats about remembering that consciousness is neither separate nor independent from the other four skandhas is well taken. Another way of saying this is that the distinction of the five skandhas are an expedient means of discussing "one mind" or "one suchness."
However, I would add another caveat that consciousness is not just "a reaction that has one of the six faculties as its basis and one of the six corresponding phenomena as its object." since that conceptualization is a construct of the four skandhas as well. The terms "six faculties" and "six corresponding phenomena" are both phrases that are the result of the first four skandhas. There is no such thing as a "phenomena" that is outside the skandhas. There is no such thing as a "faculty" that is outside the skandhas. "Phenomena" and "faculty" are third skandha appearances that become associated with the "good and bad" evaluations of the fourth skandha so that we congratulate ourselves for the good idea that consciousness is a reaction that has one of the six faculties as its basis and one of the six corresponding phenomena as its object.
To the extent that the term "consciousness only" became associated as consciousness without the appreciation of the other four skandhas the term became one-sided and thus "mind only" was necessary to point to the conscious and unconscious functions that result in our conscious awareness. Awareness of itself is crystal clear, but it is by the friction created by the polarizations of mind that light and shadow are created that then becomes developed into "self consciousness" by which we humans are able to engage in a level of awareness that we call awakening to get free from the selfishness of self consciousness and realize the consciousness of no self, no nature, no mind.
