German philosopher Arthur Schopenhauer is famous for his 19th-century contributions to the canon of Western thought, which include explorations on reason, free will, ethics, and quite a bit more. But according Schopenhauer biographer David E. Cartwright, the great German thinker also had ideas on love and marriage. They, though, were probably never destined to change the course of Western thought. It seems Schopenhauer believed in tetragamy, which says that marriage should have two men and two women. Actually, Schopenhauerian tetragamy is even weirder than that. Prolific blogger Tyler Cowen flags and reproduces from Cartwright's biography.
Tetragamy adjusted marriage into an institution that would make life better for men and women, Schopenhauer theorized, because it accommodated the natural sexual and reproductive capacities of humans in ways in which monogamy did not. It also addressed the material and financial needs of all parties in a more rational way. Two young men should marry a young woman, and when she outgrew her reproductive ability, and thereby lost her attractiveness to her husbands, the two men should marry another young woman who would "last until the two young men were old." The financial advantage of this type of marriage would be considerable, Schopenhauer thought. At first, when the two young men's incomes were low, they would only have to support one woman and her small children. Later, when their wealth increased, they would have the means to support two women and many children ... Schopenhauer never published his musings on tetragamy.
If you're curious as to why he thought this, check out Cowen's consistently interesting commenters discussing the bizarre relationships many prominent Western male philosophers seem to have had with women.