What else could justify the means other than the end? The question, though, is what end we should have in view.

In the case that you imagine, it seems that the relevant end is getting a guilty criminal in jail. Let’s assume for the sake of this discussion that we have no reason to doubt that such an end would be a good thing. Let’s assume that the person is truly guilty of a violent and unprovoked crime and that he’s very likely to continue his life of violence if left unchecked. We then weigh the good end of preventing further violence against the bad means of lying and it looks like, on balance, the goodness of the end outweighs the badness of the means. Not only, it seems, are we permitted to lie; in fact, we’re morally obligated to do so.

The problem with this line of reasoning is that it doesn’t take into account all of the relevant ends. In this context, when we are considering the ends, we must also consider the ends of the entire criminal justice system– namely, to protect the interests of all citizens, not only the interests of victims or potential victims of crimes, but also the interests of the accused. It’s very important to all of us that we have a system in place that minimizes the chances of being wrongly accused and convicted for a crime. We need to feel confident that we ourselves will not be mistakenly convicted, that our friends and family will not be mistakenly convicted, and that the truly dangerous, rather than the mistakenly convicted, will be stopped. To achieve this end, we agree to put certain rules into place which constrain the ability of any one of us to make decisions on the basis of a short-term cost-benefit analysis. It seems that overall and in the long run, we’re more likely to learn who has committed what crime if all of the witnesses are speaking the truth rather than deciding on their own whether the wrongness of deceit is outweighed by the benefits gained from conviction. For this reason, we put a rule against perjury in place and we back up that rule with a threat of punishment.

An interesting question is whether it is ever rational for any of us to consider ourselves exceptions to rules that we ourselves endorse.