Full Idea
The existence of consciousness is an absolute, because consciousness is consciousness of itself; the type of existence that consciousness has is that it is consciousness of itself.
Gist of Idea
Consciousness exists as consciousness of itself
Source
Jean-Paul Sartre (Transcendence of the Ego [1937], I (A))
Book Reference
Sartre,Jean-Paul: 'The Transcendence of the Ego' [Routledge 2004], p.7
A Reaction
I find this unconvincing. Anyone analysis the nature of the mind should think as much about animal minds as human minds. It seems obvious to me that there is likely to be an animal consciousness which is entirely of environment and its body.