Jewish Law, Oaths, and Expensive Religious Rules
His (interesting) answer to that puzzle is probably somewhere in his published writings; readers are invited to find it for themselves. What reminded me of that talk was an interesting feature I noticed in my study of Jewish law—the role of oaths.
There are many situations in which, due to the lack of evidence, a legal dispute comes down to "he said/he said." The usual resolution, in Jewish law as described by Maimonides, is by an oath. One of the two parties—which one depends on the details of the case—is asked to swear that his account is true. If he is the plaintiff, he gets to "swear and take." If he is the defendant, he gets to "swear and be quit." Either way, if he swears he wins the case, if he declines to swear he loses it. The nature of the oath he must swear depends on the nature of the case. Pretty clearly, some oaths are considered more serious than others.
Why would someone refuse to swear if the result is that he loses his case? The obvious answer is that the parties to the case are believers who either feel obligated to swear truthfully, fear supernatural punishment if they swear falsely, or both.
A "suspect party" is someone who is not permitted to swear and so, in such a situation, automatically loses. One reason to be suspect is that you have sworn in the past to something that later turned out to be false. Another is that you been observed in the past to violate religious rules—for example by eating food that was not kosher.
This suggests a function for religious rules that impose costs. They act as a filter, a signal, a way of distinguishing people who really believe in the religion from ones who don't. Knowing whether someone really believes in the religion can be valuable information. It tells you, for instance, whether to believe his oaths.