What Sorts of Things have Consciousness?
One of the most daunting aspects of the problems of consciousness is that, apparently of necessity, I have only a single data point to work with: I know that I am conscious (in our sense of the word), and I know what my conscious experience is like, but that's all. While you can tell me that you are also conscious, and you can try to communicate to me what it is like to be you, I can never have any direct experience of anyone else's consciousness, and I know of no instruments or experiments that I can use to study, or even confirm the actuality of, any consciousness but my own.
It is, that is to say, difficult to conceive of any proof that solipsism is false. But it is equally hard to imagine a proof that it is true! So we will consider, without choosing among, a number of possible answers to the question of which objects or systems in the universe have subjective consciousness.
Solipsism: (note on the term) I am conscious, and no one (and nothing) else is. This is a rather radical thesis, but perhaps not as radical as it appears at first glance. It has little or no impact on physics, for instance, or on much of anything else except perhaps morality. If I believe that only I have inner consciousness, and my morality is based on respecting the rights only of beings with inner consciousness, I may not feel morally beholden to anyone but myself; on the other hand, those non-conscious citizens and police officers out there will still react in relatively predictable ways if I attempt to exploit this lack of moral obligation.
Solipsism has some hard problems to solve. How did I end up conscious, when no one and nothing else did? What is special about me, that I should be unique in the universe? These can be treated as challenges, though, rather than objections.
Of course, if solipsism is true, it is true only for me; anyone else who believed it of themselves (to the extent that nonconscious persons can have beliefs) would be mistaken. If solipsism were true for me, and some other person were to come to believe that I am the only conscious being in the universe, that belief would be true! (What's the word for a person who believes that someone else is the only conscious being in the universe?)
Solipsism has some other consequences: if no human is conscious but me, for instance, it must be possible to do all the things that (those other) humans do, without being conscious. Consciousness must not be a necessity (and probably not even a material advantage) for survival in the evolutionary niche that humanity occupies. In this respect, solipsism is similar to any theory that holds that consciousness does not significantly affect the physical world (see How does consciousness affect the physical world?). Of course, it is possible that my consciousness does affect the physical world, even if only I am conscious; in that case, my body is a single consciously-piloted vessel in a sea of automata.
Panpsychism: Essentially everything in the universe is conscious. There are many forms of this answer. Teilhard de Chardin and Chalmers, is it?, for instance, hold that every bit of matter has some