from one's own perspective, you only know for sure that you exist and that you have a spirit. because other humans look like you and act a similar way, you assume that they feel like you and are conscious as well. but you don't know it. who says all your surrounding isn't made up by your brain/soul/consciousness? there's a nice philip k dick short story about a humanoid robot who discovers what he really is and that only he exists, and finally he decides to cut his data stream, becoming real in the very moment of his death, and making the others fade away ("the electric ant").