When some principle requires us to act in some way, this principle’s acceptability cannot depend on whether such acts are often possible. We cannot defend some principle by claiming that, in the world as it is, there is no danger that too many people will act in the way that this principle requires.

Derek Parfit, ‘Justifiability to Each Person’, Ratio, vol. 16, no. 4 (December, 2003), p. 387