This paper is about sex. And if it is about sex, it is about number. In the final weeks of the Seminar entitled Crucial Problems for Psychoanalysis Lacan identifies what has been a theme, perhaps the major one, of that year: they have been exploring what he terms the subjective positions of being. He sets up this triadic schema to help orientate his listeners: KNOWLEDGE : SUBJECT : SEX. What we have here is a very simple version of a schema that Lacan goes on to develop over the course of seven or eight weeks. He relates this triadic formulation to his other triad, the Real, the Symbolic and the Imaginary. He refers it to his topographical tool, the Moebius strip. He brings in his reading of Descartes's cogito; he unravels a theory of game he finds in Pascal; he refers to the biology of uni-cellular organisms which inhabit a twilight zone between plant and animal life. However, during these weeks the most repeated reference is to Plato. It is with one reference to Plato that this paper will concern itself. What use, what good is it for Lacan's endeavour?