Imagine living in a world where you're the only human and there are no reflective surfaces.

How would you know you had a head? You would see through your eyes, breath and eat with your mouth, touch your hair and face, etc. But you would have no "proof" your head existed because while you had constant direct experience of your head performing its functions, you would have no 3rd party objective confirmation that you had a head at all.

How does the knower know itself outside of its experience?