Suppose my wife tells me to ensure that my son brushes his teeth. I go to his bathroom and see him brushing his teeth. I did not bring it about that he brushed his teeth. Did I ensure it?
I may or may not have. I might have ignored my wife's request and just happened to go to my son's bathroom to fill a bottle of water. Or her request might have simply triggered a curiosity about my son's brushing habits. In those cases, I did not ensure it.
What needs to be the case for me to count as having ensured that he brushed his teeth? Maybe it's some kind of a disposition to make him brush his teeth if he does not do so on his own. But even such a disposition is not quite enough. Suppose, for instance, I am a domestic tyrant and I enjoy making people do things. I go to my son's bathroom quickly with a hope that I will get there before he brushes his teeth, so I will have an opportunity to make him brush his teeth. But alas he has foiled me: he already started and by the time I open my mouth in command, he has finished. In this case, too, it seems incorrect to say that I ensured that he brushed his teeth. For if I ensured that he brushed his teeth, then I succeeded at ensuring that he brushed his teeth. But in this case there is no plan of action that I succeeded at—in fact, I failed. (Note: I am talking of here of intentional ensuring. We also sometimes speak of some action unintentionally ensuring a result. In that case, "ensuring" just means something like "causally necessitating".)
For me to count as having ensured that he brushed his teeth, his brushing has to be according to my plan. Thus, I need to form a plan that he brush his teeth, and a part of that plan is the forming of a disposition to make him brush his teeth if he doesn't do so on his own, but the plan's goal needs to be that he brush his teeth rather than that I make him brush his teeth. Embarking on this plan is a genuine action on my part, an action whose end is that he brush his teeth. When I embark on the plan, I form a disposition to make him brush his teeth if he doesn't do so on his own, but that is not all that happens.
Why does this matter?
Well, consider this famous case of Rachels:
Jones also stands to gain if anything should happen to his six-year-old cousin. Like Smith [who drowns his cousin], Jones sneaks in planning to drown the child in Ills bath. However, just as he enters the bathroom Jones sees the child slip and hit his head, and fall face down in the water. Jones is delighted; he stands by, ready to push the child's head back under if it is necessary, but it is not necessary. With only a little thrashing about, the child drowns all by himself, "accidentally," as Jones watches and does nothing.Rachels thinks that this case shows that the distinction between killing and letting die is bogus. Jones is morally on par with Smith.
Rachels is probably right. But the reason for this isn't that there is no morally salient distinction between killing and letting die. It is, rather, that there is no morally salient distinction between killing and ensuring death. What Jones does is ensure death. This is a genuine action on his part. He forms a series of dispositions in himself aimed at ensuring death. This is just as much an action of his as it would be an action to program a robot to watch the child and drown him if the child didn't drown on his own. And Jones succeeds at ensuring death: he doesn't just attempt to ensure death, but he succeeds.
The death of Jones' cousin is according to his plan, albeit not his original plan, but the revised one he forms when he enters the bathroom. Compare this case. Jones comes into the bathroom. He sees his cousin drowning. He has a failure of nerve and gives up on his plan. (It doesn't matter if the failure of nerve comes after or before the observation of the drowning.) But he still doesn't go to the trouble of rescuing his cousin, which he easily could do, nor does he turn on the music to drown out the noise of the drowing lest someone else come to the rescue, though he does hope the cousin will drown. He is a wicked man, but he hasn't ensured his cousin's death.
The moral difference between watching the cousin die and ensuring death is slight in the above case, but it could be greater if Jones' reasons were different. Suppose, for instance, that upon entering the room Jones has a change of mind due to fear of getting caught. But he also notices that his cousin is a carrier of a disease that will kill Jones if Jones touches the cousin, and it is this that now is the primary reason why Jones does not pull out his cousin. Jones had a change of mind but no great change of heart. He still hopes his cousin drowns and is glad he does. But at this point, Jones' actions and inactions in the bathroom are morally defensible (though his action of going to that bathroom in order to ensure his cousin's death is not defensible). (Cf. Ian Smith's paper.)
If I am right, then when thinking about killing and letting die, we need to distinguish letting die proper from ensuring death.