Consequentialists typically think that the moral quality of one's conduct depends on the difference one makes. But consequentialists may also think that even if one is not making a difference, the moral quality of one's conduct can still be affected by whether one is participating (even if only ineffectually, or redundantly) in an endeavour that does make a difference. Derek Parfit discusses this issue – the moral significance of what I call ‘participation’ – in the chapter of Reasons and Persons that he devotes to what he calls ‘moral mathematics’. In my paper, I expose an inconsistency in Parfit's discussion of moral mathematics by showing how it gives conflicting answers to the question of whether participation matters. I conclude by showing how an appreciation of Parfit's error sheds some light on consequentialist thought generally, and on the debate between act- and rule-consequentialists specifically.