He gave historian Mary Beard one account in 1936, and repeated it in similar terms to other correspondents. Writing in response to a decision striking down a minimum wage statute, Stone said: "The business of writing dissenting opinions seems to be a form of self-expression and, like others of the type, is chiefly valuable for its subjective effect. I find that I rest better after writing them, which perhaps is a sufficient justification." Sometimes he said that he "slept better" after dissenting.

Probably the most charming version of this account was in a letter to Felix Frankfurter, at the time a Harvard law professor and later one of Stone's colleagues on the Court. "The way to happiness and a good night's sleep," Stone told Frankfurter in 1933, "is at least to agree with yourself."

Again, though, you can agree with yourself simply by noting that you dissent from the majority's decision. (And, indeed, once the liberals took control of the Court after 1937, the Court's remaining conservatives tended to adhere to the older practice of merely noting their dissent, without writing extensive decisions.)

Another letter to Frankfurter written a bit earlier offered a better reason for writing a long dissent. It might, he told Frankfurter, "inspire a little caution on the part of some of my brethren, but not many." In its strongest form, this explanation might be that an extensive dissent might persuade some other justices to jump ship – that is, to switch sides and create a new majority for the "dissenter's" opinion. And then the dissent might be converted overnight into a majority opinion. Yet, as Stone's comment also indicates, that rarely happens, and sometimes when it does you have to cement the defector's vote by assigning the opinion to him or her, and swallowing your dissent, leaving it in the archives for historians to discover.

Finally, a letter to Young Smith, a former colleague of Stone's on the faculty of Columbia Law School, gives what has become the standard explanation for dissenting. Written in the aftermath of a major Supreme court decision striking down an important component of Roosevelt's New Deal agenda, Stone's letter said, "Whether it will do any immediate good or not is probably doubtful, but I still have faith in the ultimate triumph of reason dealing with great problems of government."

Here Stone is saying that his dissent is an appeal to the future, when reason will ultimately triumph. The puzzle here is this: If indeed reason will triumph, it will be because the reasons are good ones. And, because they are good ones, judges in the future should be able to find them on their own – they will not need any special guidance from the past, that is, from the dissenting judge years before.

Another possibility, but a disquieting one, is that dissents are appeals not to the future but to today's public. By telling the public that some judges disagree with the majority, a dissent might weaken the public's support for the decision. Apart from the fact that relatively few people actually read dissents – it might be enough for the public to know that two, three, or four justices disagreed – this explanation for a long dissent might trouble those who think that judges shouldn't play a political role but should only, as Chief Justice Roberts put it at his confirmation hearings, call balls and strikes. The dissenter is then like the manager who storms out of the dugout to protest an umpire's call. It's not clear that we really want judges to behave like that.

In the end, then, it's hard to purge dissents of the taint of self-indulgence. They do let judges sleep better at night. Why the rest of us should care about that is less clear.