Abstract
However jealously we may … have defended the independence of psychology from all other sciences,… we are here overshadowed by the immutable biological fact that the living individual serves two purposes, self-preservation and the preservation of the species, which seem to be independent of each other, which we have not been able to trace back to a common source, and whose interests often conflict in animal life. Here we are realty discussing biological psychology, we are studying the psychological concomitants of biological processes.Sigmund Freud: New Introductory Lectures on Psycho-Analysis, pp. 124–125, 1949.