Full Idea
The unity of the thousand active consciousnesses through which I have added two and two to make four, is the transcendent object '2+2=4'. Without the permanence of this eternal truth, it would be impossible to conceive of a real unity of mind.
Gist of Idea
The eternal truth of 2+2=4 is what gives unity to the mind which regularly thinks it
Source
Jean-Paul Sartre (Transcendence of the Ego [1937], I (A))
Book Reference
Sartre,Jean-Paul: 'The Transcendence of the Ego' [Routledge 2004], p.6
A Reaction
This is the germ of externalism, here presented as a Platonic attitude to arithmetic, rather than being about water or gold. He claims that internalist attitudes to unity are fictions. I am inclined to think he is wrong, and that unity is biological.