Philip PettitPettit, P., 'The Consequentialist Perspective in Three Methods of Ethics (Blackwell: Oxford, 1997) pp. 163-169 replies to Singer's fourth point. For Pettit, there is a distinction between cases in which one is the only person who could possibly do anything and cases in which one is just one among millions in the same position (compare bystander effect). He argues that "There is a distinction between what it is best to do and what you cannot reasonably be denounced for doing" (p. 165).
