One objection to some solutions to the problem of evil, particularly to sceptical theism, is that if there are such great goods that flow from evils, then we shouldn't prevent evils. But consider the following parable.
I am an air traffic controller and I see two airplanes that will collide unless they are warned. I also see our odd security guard, Jane, standing around and looking at my instruments. Jane is super-smart and very knowledgeable, to the point that I've concluded long ago that she is in fact all-knowing. A number of interactions have driven me to concede that she is morally perfect. Finally, she is armed and muscular so she can take over the air traffic control station on a moment's notice.
Now suppose that I reason as follows:
- If I don't do anything, then either Jane will step in, take over the controls and prevent the crash, or she won't. If she does, all is well. If she doesn't, that'll be because in her wisdom she sees that the crash works out for the better in the long run. So, either way, I don't have good reason to prevent the crash.
- C: the value of the crash
- J: the value of my doing my job
- p: the probability that I will warn the pilots if Jane doesn't step in.
- Expected value of non-intervention: pJ+(1−p)C
- Expected value of intervention: 0 (no crash and I don't do my job).
Suppose now that I don't warn the pilots, and Jane doesn't either, and so there is a crash. Can I conclude that I did the right thing? After all, Jane did the right thing—she is morally perfect—and I did the same thing as Jane, so surely I did the right thing. Not so. For Jane's decision not to intervene may be based on the fact that her intervention would prevent me from doing my job, while my own intervention would do no such thing.
Can I conclude that I was mistaken in thinking Jane to be as smart, as powerful or as good as I thought she was? Not necessarily. We live in a chaotic world. If a butterfly's wings can lead to an earthquake a thousand years down the road, think what an airplane crash could do! And Jane would take that sort of thing into account. One possibility was that Jane saw that it was on balance better for the crash to happen, i.e., C>0. But another possibility is that she saw that C<0, but that it wasn't so negative as to make pJ+(1−p)C come out negative.
Objection: If Jane really is all-knowing, her decision whether to intervene will be based not on probabilities but on certainties. She will know for sure whether I will warn the pilots or not.
Response: This is complicated, but what would be required to circumvent the need for probabilistic reasoning would be not mere knowledge of the future, but knowledge of conditionals of free will that say what I would freely do if she did not intervene. And even an all-knowing being wouldn't know those, because there aren't any true non-trivial such conditionals.