Women, as a class, are supposed not to have hitherto been equal in intellect to men, therefore they are necessarily inferior.
 John Stuart Mill, A System Of Logic, Ratiocinative And Inductive (1843). copy citation

Context

“maxims for which the following may serve as the universal formula, “Whatsoever has never been, will never be.” As for example: negroes have never been as civilized as whites sometimes are, therefore it is impossible they should be so. Women, as a class, are supposed not to have hitherto been equal in intellect to men, therefore they are necessarily inferior. Society can not prosper without this or the other institution; e.g., in Aristotle's time, without slavery; in later times, without an established priesthood, without artificial distinctions of rank, etc.” source