Indeed, thank you @Meggers . This is another aspect of the discussion we haven’t discussed in much detail.
@yeshe.tenley I hope the parable was humorous and joyful! 
Thank you for your replies. I think it is valuable to follow through in this discussion, especially through conversation and reflection rather than sheer appeal to textual references. Here are some thoughts.
I’d like to start by clarifying some common ground. Yes, by phrases like ‘substantial,’ ‘matter-substance,’ or spacetime ‘out there’ I am generally referring to a kind of metaphysical [scientific] realism. Of course, I don’t find the term ‘scientific’ very helpful, but it should be clear what it refers to in this context.
When I mention ‘matter-substance,’ I am referring to the idea of a kind of real stuff out there in independent space-time. We can include ‘energy-susbtance’ and so on in this same category. E=MC^2 and all that. I do not think that ‘rūpa’ means “matter-substance,” either in the psuedo-scientific Abhidhamma sense (atomic elements) or the modern scientific sense of matter.
I take ‘rūpa’ to be mainly phenomenological, as the word itself suggests. ‘Rūpa’ originally refers to something like ‘appearance,’ ‘image,’ ‘sight,’ or ‘form.’ Textually speaking, it also seems to include things like an “astral” body (‘manomayā kāya’), deva-bodies, and so on. Externally, it would be the corresponding ‘form’ or appearances with physical properties we experience. So if we have a human-form, we will generally experience sights, sounds, etc. corresponding to that. Things that have characteristics of extension, space, resistance, weight, abide by physical laws such as gravity, etc. But these things as they are experienced. Not as independent, external ‘stuff’ or ‘substances’ or ‘energy’ out there. Hopefully that distinction is clear and agreed upon.
So, to me, the distinction you have made here is actually an unfair one, for lack of a better term. By “unfair,” I mean that it creates what I perceive as a false dichotomy. Namely, ‘dream form arises dependent on the mind, but form outside of dreams arises dependent upon matter, etc.’ I don’t think any form can be spoken of as arising without dependence on the mind. Do you agree? If not, I would simply ask that you describe form to me without recourse to any mental knowledge or cognition of it.
If you would agree, then I think you’ll see why I take issue with the distinction. Dream form is not unique in arising dependent on the mind, so it is somewhat misleading to form the distinction like this. I am not accusing you of trying to mislead or of bad-faith. Just how the sentence is worded. The distinction starts to lean towards that realism we mentioned.
“Form in dreams needs perception and cognition to be present, but form outside of dreams in the real world is just the interaction of matter and light and so on.” That is the type of sentiment I would disagree with.
On the other side, there is the experience of matter and light in the dream form. If, in a dream, you were in a dark room then presumably you would not be able to see forms without some kind of dream night-vision. Moreover, there is, as you say, resistance and physical properties to form in dreams just as in waking life. I also think it is plausible to imagine a dream in which one investigates the form with scientific instruments and measures laws of motion and so on.
One issue of course is that generally in dreams there is a greater sense of flexibility, control or mental influence over the scenario, and less “elegance” or symmetry in the laws governing the dream. I don’t deny this, and I’m not trying to argue that dreams and waking life are indistinguishable. But we could make reasonable counter-arguments, such as saying that the elegance or symmetry of physical laws is an arbitrary and inconclusive intuition from humans; it is not a necessity of all possible worlds. Also, in Buddhist cosmology it is said there are realms where beings do have more control such as the realms that delight in creation or that have power over other’s creations.
I agree, and I assume you would probably take no issue with at least the broad ideas painted above. That ‘form’ refers to a mere appearance that arises and ceases due to certain conditions, not a substance or essence that is made or destroyed, and that this is true of form whether in dreams or waking life. Though this does not mean we cannot use beneficial ideas and tools provisionally, nor do we need to deny basic facts about our experience.
Again, you seem to make a dichotomy here between the nature of form in dreams and the nature of form in waking life. Not a substantialist one at first glance, but I am pointing out that to me the implications are subtly substantialist. Here is what I mean.
Dream Devin’s alleged corpse does not go ‘poof.’ It refers to the manifest experience of a corpse that persists after death. If the dreamer did not wake up, then how could you make the same distinction between a dream-corpse and a waking-life corpse? If the dream did not ever end, wouldn’t the corpse just be plainly manifest, be buried, and be assumed to persist in the ground and cycle through nature as matter does? If we assume the dream has analogous properties to the human realm, I think this is reasonable.
At this point, I would ask what the word “Buddha” means etymologically. A common explanation is “Awakened One,” or “One Who Woke Up,” no? As in, from a slumber or dream (or maybe a nightmare).
What if the power of craving and ignorance kept beings locked in a cycle of dreams? We could suppose that the desire to escape would only create another dream at the end of the former, and attachment to the dream world perpetuates it. We could add in the fact that there is a relative stability in the variety of dream-realms within the larger dream-scape, and that the type of dream that follows the previous is affected by the choices, intentions, aims and actions there. Other beings are seen cycling through a similar process, ignorant and blind to it for eons. At a certain point in this thought experiment, the distinction between ‘dream world’ and ‘real world’ starts to melt away; the arising and lawful appearance of such a world in this way begins to melt notions of non-existence. Only when we can speak of a dream ending, the cessation of that world, can its existence be more readily reduced to mere conditional appearance.
So it would seem to be no surprise that waking-life matter seems endless and more substantial than dream matter, despite the conditional appearance of each being in many ways identical. It would require the waking-world to cease, like a dream ends, or someone to ‘wake up’ from it, for the same effect.
[Buddha:] “Kaccāna, this world mostly relies on the dual notions of existence and non-existence.
But when you truly see the arising of the world with right understanding, you won’t have the notion of non-existence regarding the world.
And when you truly see the cessation of the world with right understanding, you won’t have the notion of existence regarding the world.”
SN 12.15
I agree with you that ‘obstructive contact’ or ‘resistance’ occurs both in dreams and waking life. It is a characteristic of ‘form.’ But I would reiterate what I consider a false dichotomy, at least in how it is phrased. The experience of obstructive contact is just as dependent on mind in the waking world as in dream world. Do you think that when two rocks collide they experience ‘obstructive contact’? How could such a thing even be spoken of without feeling, perception, and consciousness? There is an interesting passage on just this topic. Here is an excerpt. I recommend reading the entire discourse if you get a chance!
“Suppose there were none of the features, attributes, signs, and details by which the set of mental phenomena known as name is found. Would labeling contact still be found in the set of physical phenomena?” “No, sir.”
“Suppose there were none of the features, attributes, signs, and details by which the set of physical phenomena known as form is found. Would impingement contact still be found in the set of mental phenomena?” “No, sir.”
DN 15
I’ll just note that the word translated ‘impingement’ here refers to a kind of resistance, obstructive contact, or meeting of physical phenomena. And as is stated, this could not be designated or ‘labelled’, i.e. mentally processed, without mental phenomena. These are two aspects of contact that are both equally necessary for cognizing form.
This is an important part of the discussion, to my mind. To summarize the story:
- Human Devin passes away, and the remains of their body are manifest to other humans.
- Devin is reborn with a subtle body
- Both bodies (the former human one and the celestial one) present themselves to an onlooker.
So in this story, Devin never stopped experiencing form. And the old human form still exists independently of Devin. See how that phrase “still exists” can create lurking suspicion? It just needs further clarification. So:
This means that the human-form is experienced only from the perspective of other beings, including Devin. So it is now insentient or unconscious external form. It is like the moon. A meaningful detail though is that Devin’s new celestial form would be experienced internally from their perspective. So the human-form would be external to Devin now, and it could only be designated, known, labelled, or processed with two conditions:
- Devin’s internal form offering a basis for obstructive contact with light, matter, etc., in this case the external human form.
- Devin’s internal mental faculties offering the basis for cognizing that obstructive contact.
If Devin never visited the human realm, they might have never experienced the old form externally. From Devin’s perspective, there was simply a transition in conditions, and the experience of internal-external form shifted somewhat. The fact that other beings experience their form is no different than waking up from a dream-body and having a waking-body. Maybe other beings experience the former dream-body. It would really have no relevance to the internal-external experience of the form aggregate from the perspective of the dreamer, just as would be the case for Devin once ‘waking up’ from a human-form and having a new form.
We can imagine that Devin was a pious and virtuous human. They developed their mind intent on heavenly rebirth with celestial form. If they obtain that, they would have every right to say that their previous form ceased and their new form arose. It’s a shift in conditional appearance and conditional ceasing from the particular focus on the form aggregate.
Divine Devin could be said to have ended their former human form and gained a new, celestial form. This doesn’t mean other sentient beings have to experience a sudden vanishing of the previous human form. But to suggest that ending human form must entail making the appearance of that form for all beings cease almost leans towards suggesting that Devin must make some kind of matter-substance cease. The experience of Devin’s human form is the conditional appearance of it to those beings. The experience of Devin’s divine form is the conditional appearance of it to Devin.
It would be like insisting that, upon waking from a dream, you must make the former dream-body cease to appear to all the dream-beings there. It implies that such a thing is possible or that there is some substance to be destroyed. If it appears to other dream-beings, it is because of their conditions, such as having dream-form and experiencing the dream-world. But we cannot reasonably insist, as you yourself agreed, that there is an external, substantial dream-world in dream-spacetime “out there” where the prior dream-form is persisting and appearing to dream-beings. Such a thing would be reduced to at most sheer convention.
The break-up of the aggregates does not need to imply that all perceivers see them as going poof. It simply means the arrangement of those particular conditions ceases and the arising of new conditions appear accordingly, especially internally. As in when the dream-aggregates break up and the waking-life aggregates appear. Please refer also to the thought-experiment of the perpetual, seemingly-lawful dream worlds.
By ‘truly cease’ I believe I only meant to make the distinction between the appearance of external form via internal sense contact of beings, such as seeing a corpse, and the shift in conditional appearance/cessation internally in the form aggregate, such as Devin’s metaphorical transition from human-form to divine-form. This would entail more coarse ‘human form’ ceasing and ‘divine form’ arising.
Has my response shed light on this for you? I hope it has provided more explanation and detail in what I was hoping to convey before. 
I also would like to re-iterate that it doesn’t matter (pun?) whether we are talking about “form” or “hatred.” All of this would apply, minus the particular detail of e.g. matter/light which is specific to discussing characteristics that designate ‘form.’
Suppose that there was a particular event B before which “hatred” was a possible mental state in the mind of the Buddha, and after which “hatred” ceased to appear in the Buddha’s mind. We could map it like so:
… H H H B -H -H -H …
That is, H (hatred) is present without discoverable beginning until the Buddha experiences event B (bodhi) after which H is absent without further re-arising. We could simply swap the H with F for ‘form’ and change event B to ‘P’ for ‘passing away.’ The diagram for that would look like so:
… F F F P -F -F -F …
Keeping in mind that, just as hatred may have been absent for stretches of time prior to the Buddha experiencing event B (such as if he were practicing mettā or simply not angry), so too form may be absent to him for periods of time prior to event P, such as if he were to enter deep sleep or if he were to attain a formless state of meditation, etc.
All the best in the Dhamma! 
Again, I agree that dreams can be talked about as depending upon the brain and physical processes and the physical can be talked about as depending upon the mind.
More Devins!