This paper counterbalances Lacan's dictum that desire is structured like a signifying chain by highlighting his dependence on a biological understanding of the sexual drive. Reiterating (and reinterpreting) Freud's “leaning on”—hypothesis about the genesis of desire, Lacan maintains that the sexual drive originates in the vital function. His ideas about male and female sexuality most clearly presuppose a biological understanding of the sexual drive. At various times, Lacan blurs the lines of his own conceptual difference between phallus and penis, up to the point of reducing the difference between the sexes to an opposition.