No one who has seen a baby sinking back satiated from the breast and falling asleep with flushed cheeks and a blissful smile can escape the reflection that this picture persists as a prototype of the expression of sexual satisfaction in later life.

Three Essays on Sexuality: Infantile Sexuality (1905), in James Strachey (ed.), The Standard Edition of the Complete Psychological Works of Sigmund Freud (1953), vol. 7