The distinction between killing and letting die has been a controversial element in arguments about the morality of euthanasia and physician-assisted suicide. I argue that John Mackie's analysis of causation in terms of ' inus factors,' insufficient but non-redundant parts of unnecessary but sufficient conditions, helps us to see that moral responsibility for death cannot rest on causation alone. In specifying the cause of death, some factors can be considered alternatively as either causal factors or merely parts of the presupposed background conditions. If a factor is moved from the background field into the causal field, the result is a changed background field.
Moral judgments determine how to apportion factors to the causal and background fields.
The distinction between killing and letting die has been used by many to condemn euthanasia and assisted suicide while giving approbation to withdrawing life-support systems in at least some patients. James Rachels gives the example of Smith and Jones, who stand to gain money if a child should die. If both the act and the omission are equally reprehensible, as they do seem to be, then the mere difference between killing and letting die is not sufficient to decide the morality of the case. Rachels concludes that if letting die is sometimes permissible then killing might also sometimes be permissible. There are several responses one might make.