Man is not a solitary animal, and so long as social life survives, self-realization cannot be the supreme principle of ethics.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“Egoism, at first, made men expect from others a parental tenderness; but when they discovered, with indignation, that others had their own Ego, the disappointed desire for tenderness turned to hatred and violence. Man is not a solitary animal, and so long as social life survives, self-realization cannot be the supreme principle of ethics. CHAPTER XIX Rousseau JEAN JACQUES ROUSSEAU (1712-1778) , though a philosophe in the eighteenth-century French sense, was not what would now be called a “philosopher.” Nevertheless he had a powerful influence on philosophy, as on literature and taste and manners and politics.” source