This paper is about a little known part of Allais' oeuvre, namely his restatement of the quantity theory of money. It shows that this restatement contains an original refinement of the notion of stability of the relative demand for money. To explain this refinement, this essay investigates Allais' concept of psychological time-a concept strongly emphasised but not duly examined by most of his commentators. It shows how Allais' restatement of the quantity theory amounts-in the final analysisto a theory of time. It explores an analogy, Allais mentioned, between his quantity theory and the theory of relativity in physics, revealing thereby the ontological nature of this restatement.