We donot have first hand knowledge of our self at all. We know our self as personality. This personality is given by the society. This is a second hand knowledge.
Study of scriptures tells that we are the Self. Even this knowledge is second hand.
First hand knowledge is when we realize the truth ourselves. We have always the experience of self as I exist. We donot have any doubt about that. This is also not based on sensory inputs. A pure experience of "I" consciousness without all borrowed mental images is called Aparokshanubhuti.