The Soul Hovering Over the Body, c. 1796 National Gallery of Art, Washington, CC0 (credits)

ou can't get inside anyone else's head. You have never once done it. And yet you are sure, completely sure, that there's someone in there.

...our normal waking consciousness, rational consciousness as we call it, is but one special type of consciousness, whilst all about it, parted from it by the filmiest of screens, there lie potential forms of consciousness entirely different.
   William James, The Varieties of Religious Experience (1902), Lecture XX. Project Gutenberg #621

[paraphrase, not a quotation] Nagel argued that an organism has conscious experience if there is something it is like to be that organism, and that a bat, which perceives through sonar, is a clear case where we can grant that there is something it is like, while having no way to imagine it from the inside. (after Thomas Nagel, "What Is It Like to Be a Bat?", The Philosophical Review, Oct 1974)

[paraphrase, not a quotation] Jaynes proposed that ancient people may have experienced their own thoughts as voices from outside, gods giving orders, and that consciousness as we know it was learned later, through language and culture. It is a contested idea and he knew it. (after Julian Jaynes, The Origin of Consciousness in the Breakdown of the Bicameral Mind, 1976)