 If any version of the first thesis is true, then a fortiori there is no a priori knowledge of any moral principle, since there are no true moral principles. Moreover, particularists who accept the first thesis are distinct from Rossians in that they reject the existence of true pro tanto moral principles such as that promise-keeping is a pro tanto duty. (Contemporary particularists, and moral philosophers more generally, tend to prefer ‘pro tanto’ to Ross’s ‘prima facie’, which even Ross admitted was unsatisfactory immediately upon introducing it. The problem with ‘prima facie’ is that it means at first glance, but a prima facie duty is not something that seems to be a duty at first glance, but something that would be a duty if it is not overridden in some way. ‘Pro tanto’ means to that extent, and a pro tanto duty is something that would be a duty sans phrase, if it were not overridden in some way.) Thus, particularists believe that one cannot know a priori any pro tanto principle. However, as we’ll see, these particularists nevertheless think one can attain knowledge, even a priori knowledge, of particular moral facts.