"What a strange invention marriage is " wrote Kierkegaard. "Is it the expression of that inexplicable erotic sentiment, that concordant elective affinity of souls, or is it a duty or a partnership . . . or is it a little of all that?" Like Kierkegaard a few decades later, many of Germany's most influential thinkers at the turn of the eighteenth century wondered about the nature of marriage but rejected the easy answers provided by biology...