There is no clear line which makes a sensation external or internally based. It's a messy abstraction, but there's some meaning behind it, because things can at

There is no clear line which makes a sensation external or internally based. It's a messy abstraction, but there's some meaning behind it, because things can at least be relatively sorted by how external or internally based they are. For example, you may see a spider which 99.99% of sighted individuals would also see and agree there was a spider there, or you can hallucinate one, where only you would agree about a spider being present. Clearly the first is MORE externally based than the latter.