///You can't even prove to another person that you are conscious.///

That's solipsism.

It's actually the opposite of solipsism, which is the postulation that YOU are the only conscious entity in the Universe.///

It's the same argument. If you can't prove to another that you are conscious, he can't prove to you that he is. Therefore, you are free to assume that you are the only conscious being in the universe. But then he's free to assume the same about himself.