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Thousands of species of animals probably have consciousness
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General discussions about "science" itself
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I think the main reason of the "default assumption" that only humans have consciousness is that we ourselves are human, and we are only able to prove our own consciousness. Culturally, at least in the Western worldview, humans are framed as equal. This means that we feel comfortable saying that other humans have consciousness as well, but start to get a little antsy about saying that "lesser" creatures have that same experience.
The thing is, we can't actually prove our own consciousness. I know that I am conscious, because I am me. But I can't prove that to anyone else, and no one else can prove to me that they are conscious. We use pragmatism here and make the choice to accept each other's consciousness, because if we are not pragmatic we are pulled right into an existential crisis.
We are happy to apply this pragmatic view to each other, but not to other beings. Other beings are expected to prove the impossible before we let them into the consciousness club.
The reason I say we can "prove" our consciousness is not because we can prove it to others, I mean we can prove it to ourselves because we can experience it. Anything outside our current consciousness, however, is not available with such ready belief. For everything else we must be taught to trust that it is true, even if it is impossible to have absolute proof of it.