For me, this is an easy question. I believe that animals have feelings and other states of consciousness, but neither I nor anyone else has been able to prove it. We can't even prove that other people are conscious, much less other animals.The one that really shocked me was Donald Hoffman, a cognitive scientist at UC-Irvine, who is apparently some kind of neo-Berkelean Idealist. I didn't know people like that existed. Hoffman says:
I believe that consciousness and its contents are all that exists. Space-time, matter and fields never were the fundamental denizens of the universe but have always been, from their beginning, among the humbler contents of consciousness, dependent on it for their very being. The world of our daily experience - the world of tables, chairs, stars and people, with their attendant shapes, smells, feels and sounds - is a species-specific user interface to a realm far more complex, a realm whose essential character is conscious. It is unlikely that the contents of our interface in any way resemble that realm. Indeed the usefulness of an interface requires, in general, that they do not. For the point of an interface, such as the Windows interface on a computer, is simplification and ease of use. We click icons because this is quicker and less prone to error than editing megabytes of software or toggling voltages in circuits. Evolutionary pressures dictate that our species-specific interface, this world of our daily experience, should itself be a radical simplification, selected not for the exhaustive depiction of truth but for the mutable pragmatics of survival. If this is right, if consciousness is fundamental, then we should not be surprised that, despite centuries of effort by the most brilliant of minds, there is as yet no physicalist theory of consciousness, no theory that explains how mindless matter or energy or fields could be, or cause, conscious experience.The name 'Hoffman' is somewhat familiar, and I have this idea that maybe I should've already know about him, but I didn't. The other response I wanted to draw attention to: Robert Sapolsky thinks that non-proof implies non-justification:
Mine would be a fairly simple, straightforward case of an unjustifiable belief, namely that there is no god(s) or such a thing as a soul (whatever the religiously inclined of the right persuasion mean by that word). ... I'm taken with religious folks who argue that you not only can, but should believe without requiring proof. Mine is to not believe without requiring proof. Mind you, it would be perfectly fine with me if there were a proof that there is no god. Some might view this as a potential public health problem, given the number of people who would then run damagingly amok. But it's obvious that there's no shortage of folks running amok thanks to their belief. So that wouldn't be a problem and, all things considered, such a proof would be a relief - many physicists, especially astrophysicists, seem weirdly willing to go on about their communing with god about the Big Bang, but in my world of biologists, the god concept gets mighty infuriating when you spend your time thinking about, say, untreatably aggressive childhood leukemia.This seems to be just inconsistent. He says that justification requires proof, and that he's unwilling to believe without proof, but then he goes on to admit that there's no proof against the existence of gods and souls.