"Suppose that I do apply for a job as an aid worker. How much good will I do as an Oxfam employee? Certainly I'll do some good. But in getting that job for Oxfam, the marginal difference that I will make not is all the good that I will do in that job at Oxfam. It's all the good that I will do at that job at Oxfam minus all the good that the second best applicant for the job would've done. Because if I wasn't in the field at all, Oxfam would have appointed the person they considered the best applicant, presumably the second best applicant. So that person would've done almost as much good as I would've."