F. Engels, in his book The Origin of the Family, Private Property and the State: "monogamy was the only known form of the family under which modern sex love could develop, it does not follow that this love developed exclusively, or even predominantly, within it as the mutual love of the spouses. The whole nature of strict monogamian marriage under male domination ruled this out."[25] Sigmund Freud stated, "It can easily be shown that the psychical value of erotic needs is reduced as soon as their satisfaction becomes easy. An obstacle is required in order to heighten libido; and where natural resistances to satisfaction have not been sufficient men have at all times erected conventional ones so as to be able to enjoy love.