Many philosophers believe that our ordinary English words man and woman are “gender terms,” and gender is distinct from biological sex. That is, they believe womanhood and manhood are not defined even partly by biological sex. This sex/gender distinction is one of the most influential ideas of the twentieth century on the broader culture, both popular and academic. Less well known are the reasons to think it’s true. My interest in this paper is to show that, upon investigation, the arguments for the sex/gender distinction have feet of clay. In fact, they all fail. We will survey the literature and tour arguments in favor of the sex/gender distinction, and then we’ll critically evaluate those arguments. We’ll consider the argument from resisting biological determinism, the argument from biologically intersex people and vagueness, the argument from the normativity of gender, and some arguments from thought experiments. We’ll see that these arguments are not up to the task of supporting the sex/gender distinction; they simply don’t work. So, philosophers should either develop stronger arguments for the sex/gender distinction, or cultivate a variety of feminism that’s consistent with the traditional, biologically-based definitions of woman and man.