Sartre's recognition of the philosophical impasse produced by any attempt to derive prescription from description does not prevent his being tempted by such a move. What is may not provide any rules for what ought to be, but slippage between the two domains is not only common, it is inscribed in the very terminology of moral debate: both the Greek term (ethics) and the Latin term (morals) register the age-old tendency to slide from a simple statement of affairs to a prescription for conduct: the ethos (nature, disposition) and the mores (customs) become enshrined not merely de facto but also de jure.

Sartre is fully conscious of the dangers of such a shift. Moreover his refusal of an essential human nature might seem to render inappropriate, or even impossible, any attempt to ground a universal ethics. Despite this – or perhaps because of it – Sartre's whole work can be read as a long meditation on moral questions and dilemmas. Of course he never published a major ethical study, though three were projected: one to follow L'Etre et le Néant, one subsequent to the Critique de la raison dialectique, and a final version at the end of his life in conjunction with his friend Benny Lévy.