>I'm going to disagree that the point of prison (as punishment) is cruelty. It absolutely can work without it,

This is an absurd claim. How does punishment work without cruelty? Remember, that I allow for people to disagree on the quantity of cruelty (supposing that can be quantified)... so you're proposing cruelty-free punishment. This misses the entire point of what punishment is. Perhaps your reading comprehension is faulty, and you confuse the word with "rehabilitation". I see that alot.

>but I'll recognize this is nuanced and there are likely those that can't be rehabilitated effectively.

Hah! I hadn't even read that sentence yet when I wrote the above! Damn I'm good. Rehabilitation is science fiction. No one has even ever proposed a theory on how or why it's supposed to happen, let alone engineered a method to induce it to occur. On reddit, these conversations are often hilarious, but maybe here on Hackernews it will be glorious. Why do you think you need to do to cause a violent criminal to rehabilitate? If the method is performed diligently, what percentage of violent criminals rehabilitate, and what sort of timeline are we talking? Why do the failures (and let's be honest, there are failures in every manufacturing process) fail? Can these be sorted out beforehand?

But at the end of the day, there is still a big problem. Justice is what society promises to the victims so that they will refrain from revenge; the state must carry out the revenge for them. It does so for several reasons: to discourage eternal feuds, to ensure that the correct culprit is targeted, and to avoid normally nonviolent people from growing accustomed to violence. To satisfy their monkey need for vengeance in the most civilized manner possible. If you fail to satisfy it, then these people are obligated to do it themselves. Rehabilitation alone can't substitute, even if it were possible. For the lesser criminals, incarceration suffices. For those more depraved, we don't want them rehabilitated. It serves no need at all.