So if there existed a hypothetical institution with the power to mete out preventive imprisonment, and which would reliably base its decisions on mathematically sound consequentialist arguments, would you be OK with it? I'm really curious how many consequentialists here would bite that bullet. (It's also an interesting question whether, and to what extent, some elements of the modern criminal justice system already operate that way in practice.)

[EDIT: To clarify a possible misunderstanding: I don't have in mind an institution that would make accurate predictions about the future behavior of individuals, but an institution that would preventively imprison large groups of people, including many who are by no means guaranteed to be future offenders, according to criteria that are accurate only statistically. (But we assume that they are accurate statistically, so that its aggregate effect is still evaluated as positive by your favored consequentialist calculus.)]

This seems to be the largest lapse of logic in the (otherwise very good) above post. Only a few paragraphs above an argument involving the reversal test, the author apparently fails to apply it in a situation where it's strikingly applicable.

The only problem with this is that it works in reverse. We could put people who haven't commited a crime in jail on the grounds that they are likely to or it helps society when their in jail.

Once you factor in the dangers of giving humans that sort of power, I think that "problem" goes away for the most part.

I think a lot of you are missing that (a version of) this is already happening, and the connotations of the words "jail" and "imprison" may be misleading you.

Typically, jail is a place that sucks to be in. But would your opinion change if someone were preventatively "imprisoned" in a place that's actually nice to live in, with great amenities, like a gated community? What if the gated community were, say, the size of a country?

And there, you see the similarity. Everybody is, in a relevant sense, "imprisoned" in their own country (or international union, etc.). To go to another country, you typically must be vetted for whether you would be dangerous to the others, and if you're regarded as a danger, you're left in your own country. With respect to the rest of the world, then, you have been preventatively imprisoned in your own country, on the possibility (until proven otherwise) that you will not be a danger to the rest of the world.

(A common reason given for this general restriction on immigration. though not stated in these terms, is that fully-open borders would induce a memetic overload on the good countries, destroying that that makes them worthy targets of immigration. So indeed, a utilitarian justification is given for such preventative imprisonment.)

Again, the problem is recognizing what counts as a "prison" and what connotations you attach to the term.

Great point. in real life one should usually look for the best available option when considering a potentially costly change rather than just choosing one hard contrarian choice on a multiple choice test. The fact that we have conflicting intuitions on a point is probably evidence that better 'third way' options exist.

Before things go that far, shouldn't a society set up voluntary programs for treatment?

Who would volunteer to go to jail? Seriously, if the cops came to your door and told you that because your statistics suggested you were likely to commit a crime and you had to go to a "rehabilitation program", would you want to go, or resist (if possible)?

Exactly how does one draw the line between punishment and treatment?

From this, hypothetical, point of view, there is no difference. There is no real punishment, but you can hardly call sending someone to jail or worse, execution, treatment.

"consequentialists don't believe in punishment for its own sake, they do believe in punishment for the sake of, well, consequences."

Exactly how does one draw the line between punishment and treatment? Our society has blurred the two notions.

Very true. As I noted in my other comment, jails necessarily suck to be in, above and beyond the loss of freedom of movement.

We just don't have a common, accepted protocol to handle people who are "dangerous to others, though they haven't (yet) done anything wrong, and maybe did good by turning themselves in". Such people would deserve to be detained, but not in a way intended to be unpleasant.

The closest examples I can think of for this kind of treatment (other than the international border system I described in the other comment) are halfway houses, quarantining, jury sequestration, and insane asylums (in those cases where the inmate has just gone nuts but not committed violent crimes yet). There needs to be a more standard protocol for these intermediate cases, which would look similar to minimum security prisons, but not require you to have committed a crime, and be focused on making you less dangerous so you can be released.

If it really does help the society, it's by definition not a problem, but a useful thing to do.

I suppose so, under this point of view, but does that make it right? Also note that "helping society" isn't an exact definition. We will have to draw the line between helping and hurting, and we have already done that with the constitution. We have decided that it is best for society if we don't put innocent people in jail.