Due to his initial training and his project for a scientific (or metapsychological) psychology, Freud developed a biological way of thinking throughout his work. Three stages in this way of thinking can be described. The first conceives the mind in relation to the brain; it is the closest to neuroscience today. The whole of the first topography and all the theorizations relating to mental functioning are derived from it. The second is based on human sexuality and its particularities, notably its diphasic character, making it possible to understand what Freud ultimately understood by the drive, namely, the search for an object outside the subject's own body. The third is inaugurated by the vast speculation on the death drive and, regarding the life principle, raises the question as to how far the equilibrium between construction and destruction is essential to the perpetuation of life itself, since the notion of Eros becomes the very principle that makes it possible to hold these two opposing tendencies together.