Some arrangements of particles feel better than others. Why?
We have no general theories, only descriptive observations within the context of the vertebrate brain, about what produces pain and pleasure. It seems like there's a mystery here, a general principle to uncover.
Let's try to chart the mystery. I think we should, in theory, be able to answer the following questions:
(1) What are the necessary and sufficient properties for a thought to be pleasurable?
(2) What are the characteristic mathematics of a painful thought?
(3) If we wanted to create an artificial neural network-based mind (i.e., using neurons, but not slavishly patterned after a mammalian brain) that could experience bliss, what would the important design parameters be?
(4) If we wanted to create an AGI whose nominal reward signal coincided with visceral happiness -- how would we do that?
(5) If we wanted to ensure an uploaded mind could feel visceral pleasure of the same kind a non-uploaded mind can, how could we check that?
(6) If we wanted to fill the universe with computronium and maximize hedons, what algorithm would we run on it?
(7) If we met an alien life-form, how could we tell if it was suffering?
It seems to me these are all empirical questions that should have empirical answers. But we don't seem to have much for hand-holds which can give us a starting point.
Where would *you* start on answering these questions? Which ones are good questions, and which ones are aren't? And if you think certain questions aren't good, could you offer some you think are?
As suggested by shminux, here's some research I believe is indicative of the state of the literature (though this falls quite short of a full literature review):
Tononi's IIT seems relevant, though it only addresses consciousness and explicitly avoids valence. Max Tegmark has a formal generalization of IIT which he claims should apply to non-neural substrates. And although Tegmark doesn't address valence either, he posted a recent paper on arxiv noting that there *is* a mystery here, and that it seems topical for FAI research.
Current models of emotion based on brain architecture and neurochemicals (e.g., EMOCON) are somewhat relevant, though ultimately correlative or merely descriptive, and seem to have little universalization potential.
There's also a great deal of quality literature about specific correlates of pain and happiness- e.g., Building a neuroscience of pleasure and well-being and An fMRI-Based Neurologic Signature of Physical Pain. Luke covers Berridge's research in his post, The Neuroscience of Pleasure. Short version: 'liking', 'wanting', and 'learning' are all handled by different systems in the brain. Opioids within very small regions of the brain seem to induce the 'liking' response; elsewhere in the brain, opioids only produce 'wanting'. We don't know how or why yet. This sort of research constrains a general principle, but doesn't really hint toward one.
In short, there's plenty of research around the topic, but it's focused exclusively on humans/mammals/vertebrates: our evolved adaptations, our emotional systems, and our architectural quirks. Nothing on general or universal principles that would address any of (1)-(7). There is interesting information-theoretic / patternist work being done, but it's highly concentrated around consciousness research.
Bottom line: there seems to be a critically important general principle as to what makes certain arrangements of particles innately preferable to others, and we don't know what it is. Exciting!