Freud is not merely referring to all the activities conventionally recognized as children's play; he is also making a structural analysis of the infantile activities which he insisted were sexual and perverse, of which thumb-sucking is the prototype. In early infancy the child, according to Freud, inevitably takes his own body as his sexual object; in doing so, he plays with it. Play is the essential character of activity governed by the pleasure-principle rather than the reality-principle. Play is 'purposeless yet in some sense meaningful'. It is the same thing if we say that play is the erotic mode of activity. Play is that activity which, in the delight of life, unites man with the objects of his love, as is indeed evident from the role of play in normal adult genital activity. But according to Freud, the ultimate essence of our being is erotic and demands activity according to the pleasure-principle."
Norman O Brown Life against Death - The Psychoanalytical Meaning of History (1959) p.32-33