Suppose you are morally certain that performing a certain act will prevent a catastrophic evil which will otherwise occur.
If I'm a movie producer, and Alfred Hitchcock offers me this pitch, I'm going to say, "You interest me strangely. Go on."
But if I'm me, and someone in a discussion on torture, interrogation, and national security offers me this pitch, I'm going to say, "Why on earth would I suppose that?"
Here are three reasons why I won't suppose a ticking time bomb scenario:
1. It is impossible.
Hypothetical scenarios can be very helpful in understanding different facts of an issue.
But this scenario isn't merely hypothetical, or even counterfactual. It demands moral certainty that can only arise from knowledge of facts that cannot be known. It supposes moral certainty when moral certainty is impossible.
And once I've supposed one impossibility, I'm free to suppose anything, such as photonic quolls that come into existence merely by being imagined, and whose existence necessarily prevents all catastrophic evil.
2. It is impractical.
It's not just the quolls that make this scenario a non sequitur in the national security debate. As Zippy has pointed out, we have actual, real-world scenarios to discuss.
In the actual, real world, impossible scenarios don't occur. If we want to know what to do in the actual, real world, why don't we suppose we're inclined to suspect that performing a certain act could lead to some good but unknown effect?
This is the actual, real-world problem we're faced with, and we can't solve it if we're off worrying about imaginary, impossible problems.
3. It tells us nothing we need to know.
The scenario sets up two questions: What would you do, and what should you do?
If I'm asked the former, I'd want to know why. Am I a moral exemplar, such that if I would do something then it is necessarily good? And if I don't answer the way you want me to, do I cease to be an exemplar?
What I would do might be good, but whether it's good isn't proven by whether I would do it. The same goes for everyone else.*
If the question is, what should you do, then my answer is, "Why are we wondering about morality in impossible and imaginary situations?"
UPDATE: More on moral certainty and the ticking time bomb scenario here.
* I've noticed that when the question is, "What would you do?," the intention is often to direct a torrent of indignation at anyone who wouldn't do what other people actually did in real life. There's a similar phenomenon on the topic of the atomic bombing of Japan. In neither case is the indignation relevant to the moral issue, because, again, nobody's a moral exemplar.