The term ‘pro tanto’ may be somewhat unfamiliar (partially, perhaps, because my use of it is also slightly ungrammatical), so a word of explanation may be in order. A pro tanto reason has genuine weight, but nonetheless may be outweighed by other considerations. Thus, calling a reason a pro tanto reason is to be distinguished from calling it a prima facie reason, which I take to involve an epistemological qualification: a prima facie reason appears to be a reason, but may actually not be a reason at all, or may not have weight in all cases it appears to. In contrast, a pro tanto reason is a genuine reason—with actual weight—but it may not be a decisive one in various cases.
(Shelly Kagan, The Limits of Morality, Oxford Ethics Series, ed. Derek Parfit [Oxford: Clarendon Press, 1989], 17 [italics in original; footnote omitted])
