Panpsychism is the belief that consciousness or experience is fundamental to matter. That is, the fundamental building blocks of matter, fundamental particles, have the simplest experience. When these particles are combined in the right way into complex systems, advanced experience emerges.
Rational or extreme panpsychism though has a problem with that latter assumption. Why would only complex systems have a unified experience of self? After all, in panpsychism fundamental particles are the simplest systems and they have experience, so why wouldn’t all systems, no matter how simple, have some kind of experience?
A human can only have complex experience because the cellular systems a human is made of interact with one another. These interactions would shape or change experience in the supersystem. Our mind then is the awareness of these interactions. Or perhaps more accurately, these interactions are experience. This system awareness can only happen if each interaction of subsystems (in this case cells) is aware of itself as an interaction. Otherwise, experience would supernaturally emerge from non-experience rather than merely naturally change. Two humans interacting with one another would amount to a system which is aware of itself on some albeit primitive level as a system.
This way of looking at panpsychism resolves the combination problem, an irritating thorn in the side of mainstream panpsychism. Rational panpsychism however, works in the same way as observable physical phenomena like movement. Muscle cells working together to move your arm are in fact, that arm movement. If only two cells interact there is still a movement, simple though it may be. It would involve special pleading to proclaim that interacting neurons would involve a special case that would not involve a shared experience. So, in rational panpsychism, the experience of subsystems combine to form system awareness just as muscle cell movements combine to form muscle movements.
Since the Universe itself is a system, that system must also experience. So rational panpsychism is identical to rational pantheism.
Unfortunately for many, rational panpsychism leads to the uncomfortable conclusion that rocks, corpses and even computers experience, albeit at an extremely primitive level. Logic though, cares not for primitive appeals to incredulity. For the rational, reason trumps intellectual comfort.
Decades from now what is known as panpsychism will be this rational panpsychism. It will reign because it rejects the irrational unfounded special pleading assumption that only complex systems can experience or that experience can strongly emerge from non-experience.