Teaching Utilitarianism

A memorable thought experiment from Peter Singer's famous paper, "Famine, Affluence and Morality":

if I am walking past a shallow pond and see a child drowning in it, I ought to wade in and pull the child out. This will mean getting my clothes muddy, but this is insignificant, while the death of the child would presumably be a very bad thing.

One of my students' recollection:

...let's say there is a small child drowning in a pile of mud, and you walk by and see this. It is very easy for you to save the child, just step in the mud and grab her. However, is it of moral significance for YOU to do so... your pants will probably get ruined and maybe you think that somebody else will do it, however you have to weigh your options. Singer was trying to show that you might think, "Of course you save the child's life, it's just a pair of pants," however, those pair of pants might be of equal or more significance to the man than saving the girl's life. The principle states that it is all about your opinion and what is significant to you.

What the hell am I doing wrong!?

At least when students badly misunderstood philosophy of science I could tell myself, "They just don't get it because it's a bit technical. They're not bad people!" I'm not looking forward to discussion of the death penalty later in the semester.