 One influential building-block for thinking about moral conflicts is W. D. Ross’s notion of a “prima facie duty”. Although this term misleadingly suggests mere appearance – the way things seem at first glance – it has stuck. Some moral philosophers prefer the term “pro tanto duty” (e.g., Hurley 1989). Ross explained that his term provides “a brief way of referring to the characteristic (quite distinct from that of being a duty proper) which an act has, in virtue of being of a certain kind (e.g., the keeping of a promise), of being an act which would be a duty proper if it were not at the same time of another kind which is morally significant.” Illustrating the point, he noted that a prima facie duty to keep a promise can be overridden by a prima facie duty to avert a serious accident, resulting in a proper, or unqualified, duty to do the latter (Ross 1988, 18–19). Ross described each prima facie duty as a “parti-resultant” attribute, grounded or explained by one aspect of an act, whereas “being one’s [actual] duty” is a “toti-resultant” attribute resulting from all such aspects of an act, taken together (28; see Pietroski 1993). This suggests that in each case there is, in principle, some function that generally maps from the partial contributions of each prima facie duty to some actual duty. What might that function be? To Ross’s credit, he writes that “for the estimation of the comparative stringency of these prima facie obligations no general rules can, so far as I can see, be laid down” (41). Accordingly, a second strand in Ross simply emphasizes, following Aristotle, the need for practical judgment by those who have been brought up into virtue (42). 