Consider—as an analogy—the reasons associated with promise-keeping. If I
have already promised to meet with a student to discuss his paper at 12.30 p.m.,
then I may not accept an invitation to lunch with a colleague at that time. There
are good reasons not to inconvenience my student or disappoint his expectations,
and those reasons outweigh the reasons associated with lunch. So far so good.
But then what if I find out that it is going to be a really delicious lunch (which
I did not know when I conceded that the obligation to the student “outweighed”
the lunch invitation)? Does the introduction of this new factor change the
balance? Not at all. The attractions of lunch and the importance of meeting my
student are not to be weighed against one another, once the promise has been
given. The existence of the promise provides a reason for not acting on
considerations like the quality of the lunch; it provides what Joseph Raz has
called an “exclusionary reason.”5