Excellent question.(Commentary) Sensation and ideas each constitute a separate skandhas: all the other mental dharmas are placed within the samskaraskandha. Why is this?
(The commentary explains this further, but before we get into that, I previously thought that roots of dispute referred to vivadamula, which in the vivadamula-sutta refers to six causes of discord in the community, namely people who are angry, mean, envious, crafty, with wrong views or attached to their own views. But here is means something different.)21. The two mental states, sensation and ideas, are defined as distinct skandhas because they are the causes of the roots of dispute, because they are the causes of transmigration, and also by reason of the causes which justify the order of the skandhas.
So the “two roots of dispute” here are attachment to pleasure and attachment to ideas. So sensation and ideas are the causes of this. They also cause transmigration, in that desire for sensation and attachment to wrong views cause that. The section below (22b-d) explains the third comment about the order of the skandhas.
They cannot be placed within any of the five skandhas – obviously. And one cannot fabricate a #6 skandha to contain them, since it would be incoherent as a category, and thus violate the definition given above of ‘skandha’. Moreover the point of the skandhas is to define everything that causes or purifies defilements, which the unconditioned dharmas do not do.(Commentary) why do unconditioned things, which form part of the dharmayatana and the dharmadhatu, not form part of the skandhas?
22a-b. Unconditioned things are not named with respect to the skandhas, because they do not correspond to the concept.
And I thought it was just arbitrary…but of course nothing is arbitrary here.(Commentary) We have defined the skandhas. We should now explain the order in which the skandhas are enumerated
So matter is the grossest, since it is subject to resistance. Then we proceed to the non-material skandhas, and the grosses is sensation, because it requires physical contact to provoke it; then the commentary asserts that ideas are grosser than formations and cognition. So you have a sequence.22b-d. The order of the skandhas is justified by their grossness, their defilement, the characteristic of the jug, etc., and also from the point of view of their spheres of influence.
The commentary then relates this to defilement. Men and women are attracted by their bodies (rupaskandha) because they produce pleasant sensation (vedanaskandha), which gives rise to wrong ideas due to their dispositions, which then defiles their cognition. This seems a bit of a stretch once you get into the non-material skandhas but…it does make sense.
Then the commentary offers another nice analogy, of the jug (or rather, bowl or pot - I struggle to see this as a jug unless we’re talking about gazpacho).
It then relates it to the spheres of existence:(Commentary) Matter is the pot, sensation is the food, ideas are the seasoning, the samskaras are the cook, and the mind is the consumer.
This feels a bit less persuasive – I understand the first two but the division of Arupadhatu into these categories such that it fits with the scheme of the skandhas seems a little contrived. Or is that just my reading of it?(Commentary) …one sees that Kamadhatu is characterized by matter, namely by the five objects of sense enjoyment. … Rupadhatu, that is to say the four dhyanas, is characterized by sensation (organs of pleasure, satisfaction and indifference). The first three stages of Arupadhatu are characterized by ideas; ideas of infinite space, etc. The fourth stage of Arupadhatu, or the summit of existence, is characterized by volition, the samskara par excellence, which there creates an existence of twenty-four thousand kalpas. Finally, these diverse stages are the ‘abodes of consciousness’: it is in these places that the mind resides. The first four skandhas constitute the field; the fifth constitutes the seed.