I'll take for granted three things:
- Long-term incarceration for serious crimes is permissible
- Income tax, at roughly the level of taxation in the U.S., is permissible (though there may be particular features of the present U.S. tax code that are unjustifiable)
- Torture is wrong.
Let's add some further, plausible claims. Some things may be wrong to do due to some complex moral reasoning which shows that even though the action does not prima facie seem to harm anybody, nonetheless the action is wrong (contraception is like that). But some things are wrong for a very straightforward reason: they are wrong because of the clear and obvious harm they impose on the victim. Torture seems to be one of those things:
- Torture is wrong because of the harm imposed on the victim.
- If an action is wrong because of the harm imposed on the victim, then an action which imposes a greater harm under the same circumstances on the same victim will also be wrong.
- If a self-interestedly rational and well-informed person would prefer B to A, where B is harmful, then it would be more harmful for her to receive A instead of B.
- Some self-interestedly rational and well-informed persons would prefer some (perhaps moderate) instances of torture to a lifetime of taxation (at the level of U.S. income taxes) or to long-term incarceration.
I think (1)-(3) are correct. I also think (7) is true. We would not think that someone who endured severe pain for, say, 15 minutes in the course of escaping from a twenty-year jail sentence was self-interested irrational. According to some stuff I found online, the average American in 2004 paid $9377 in income taxes. If this amount were annually invested at 8% (which is I think fairly conservative for such a long-term investment), in 40 years, it would yield $2.4 million. We would not think that someone who ran through non-life-threatening but very painful flames in order to get to a treasure chest containing $2.4 million, even if the chest could only be opened in 40 years, would be irrational in so doing.
So, we need to reject (4), (5) or (6) to get out of the difficulty. Of these claims, I find (5) the most plausible. So that leaves (4) and (6) as candidates for rejection. I think (6) is a bit more plausible than (4), though I am suspicious of the whole concept of self-interested rationality. If so, then (4) should be rejected.
But if torture is not wrong because of the harm inflicted to the victim, what makes it wrong? I am inclined to say the following: It is wrong because to torture someone is unloving, and the duties of love are the whole of the moral law. And it is unloving not just because of the harm inflicted on the victim, because there is more to being loving than providing benefits and more to being unloving than inflicting harms. Love is a unitive relationship, and acts that are innately counter-unitive, such as torture, marital contraception, or lying (I am not putting them all on an equal moral footing—equally, they are wrong, but they are not equally wrong, if you get my drift), are also wrong.
A different way of rejecting (4) might be given by a Kantian.