Suppose I design a Morality Room to test your feelings about the morality of certain actions. In the room, I screen a movie showing an act, the consequences of that act, and the consequences of not acting. You then press the "Good" or "Evil" button to indicate whether you feel the act was good or evil.
So, when I show you a movie of your foiling a handbag-snatching attempt, and this leading to the thief's immediate arrest (instead of his striking again), you press the "Good" button.
[We can imagine variations on this theme, with movies showing multiple actions and scoring them each relative to one another. For now, the simple version will suffice.]
The Morality Room is a useful tool for illustrating how we can be confused when we consider counterfactuals.
The classic one is "how would you feel if you were aborted and never existed?" The movie shows you the alternatives, namely life as you know it, then life without you. By now, you have probably noticed the problem. You cannot objectively answer whether your non-existence was good, when you are the one existing to provide the answer. Your answer presumes your existence.
Another question is "would it be okay if you were murdered, assuming that you did not suffer, and the memories of others were erased to ensure you did not suffer?" Or, equivalently, "would it be okay if you ceased to exist if no one suffered directly on account of your non-existence?"
Again, this question cannot be answered by the room. You, the judge, will suffer when you see the movie, so you must answer that the act is evil.
What can we take from this? Well, I'm not saying that we ought not think that "disappearing" people, even painlessly, is not evil. It feels pretty evil to me. Instead, what we learn is that we cannot rationally reach the conclusion that an act is objectively good or evil when we are hypothetically indifferent to the act. We cannot objectively say that murder is wrong even when it happens to someone we don't know, when there is no suffering, and when we are unconscious of the event. It's too late, because we are already in the room and we've seen the movie. We can't be unconscious of the event and still answer the question.
This isn't to say that subjective morality isn't perfectly adequate. It is.
P.S. I can't have been the first person to think of this thought experiment. Anyone know of a reference to something more original?