A common defense of an unethical act is to say, "If I didn't do it, someone else would." Let's say for the sake of argument that such a claim is true. Is it a credible defense? I wonder if a utilitarian in particular should be receptive to this line of thought.

Good question. Yes, utilitarianism is a form of consequentialism and, as the term suggests, the concern is with the net consequences of action. If some consequence is inevitable (imagine this involves an injury) and it cannot be prevented, then it may be an open question about who brings about the consequence. I note that this would be an "open question," because a utilitarian might still have good reasons to be very concerned about who does the act. Let's say you and I have applied for a job that will result in one of us (unfairly) injuring someone and, for some reason, this unfair injury cannot be prevented. I get the job and reason that, well, if I don't do it, you would. It still may be worse for me to do the act for, having done it, perhaps I have a weak character and am more likely to do far more unjust acts (than you). But, setting aside this additional way of measuring and comparing consequences, it is usually the non-utilitarian (the Kantian or advocate of virtue theory) who claim that it matters a great deal who actually does the harm. For some, it is a matter of integrity. For a classic case for integrity do a search for Bernard Williams on integrity against utilitarianism.

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