For Kant, Aristotle's categories are arbitrary but brilliant and they do not ultimately correspond to extramental reality. For Aquinas, however, they are rational divisions of extramental being. In this perennial and ongoing dispute, the various positions seem to dissolve upon delving into the particulars of any one category. If, however, the categories are divisions of extramental being, it should be possible to offer plausible accounts of particular categories. I offer Aquinas's unstudied derivation of quality as a test case to see (...) how one could hold, and how Aquinas did hold, to a realism about Aristotle's categories at a highly specific level. Although Aristotle divides quality into four species and some further subspecies, unlike Aquinas, he offers no reasons for these divisions. For Aquinas each accident is a particular mode of existing, that is, it is a particular way that an accident exists in a substance. In the case of quality, this mode of existing follows substantial form and its real extramen.. (shrink)