Full Idea
'To know oneself well' is inevitably to look at oneself from the point of view of someone else, in other words from a point of view that is necessarily false.
Gist of Idea
Knowing yourself requires an exterior viewpoint, which is necessarily false
Source
Jean-Paul Sartre (Transcendence of the Ego [1937], II (D))
Book Reference
Sartre,Jean-Paul: 'The Transcendence of the Ego' [Routledge 2004], p.38
A Reaction
(This is because the Ego cannot be known from the outside). I agree with Russell that the self is most evident when we are engaged with the world, which implies that you can only acquire self-knowledge by studying those engagements.
Related Idea
Idea 19249 'Know yourself' is not introspection; it is grasping how others see you [Peirce]