An argument by David Hume against the immortality of the soul:
On the theory of the soul's mortality, the inferiority of women's capacity is easily accounted for. Their domestic life requires no higher faculties either of mind or body. This circumstance vanishes and becomes absolutely insignificant on the religious theory: the one sex has an equal task to perform as the other; their powers of reason and resolution ought also to have been equal, and both of them infinitely greater than at present. (Essay on the Immortality of the Soul)
Talk about one man's modus ponens being another man's (or woman's!) modus tollens! There has recently been some interesting work by feminist philosophers on the history of philosophy, in which it is critically examined whether the absurd remarks about women made by so many male philosophers is dependent on or consistent with their philosophical principles, and (simultaneously) how those principles are actually or potentially conducive to women's equality and serious thought about women's issues. It's been very uneven; in some cases, as in Augustine or Aquinas, it has been rather poor. Things have been attributed to Augustine, for instance, that he explicitly opposes. Nonetheless, it is an important and valuable sort of work; and I look forward to it continuing. One philosopher who has certainly not yet had his comeuppance, with regard to both his sexism and his racism (I think it can be shown that neither are inconsistent with his moral principles), is Hume. There also needs to be some reclamation of egalitarians, like the unfortunate Beattie, who, despite his adamant insistence on fundamental racial and sexual equality (and severe and extensive criticisms of Hume on the former), has gone down in philosophical history as "that bigot fellow, Beattie." Why? Because that's what he was called once by Hume.