"Why is it reasonable to believe in prisons, but not in the hell?"

[FlatAssembler]

No, because those things do not seem to contradict the basic principle of social sciences, that is the principle of rationality. One can rationally believe hospitals, schools, public libraries and perhaps even police stations are a good thing. But one cannot rationally believe prisons are a good thing. And for prisons to exist, politicians would need to be systematically biased (all biased towards believing prisons are a good thing), which contradicts the principle of rationality.

Can you clarify just what your argument is about? Is it whether prisons exist or not, or prisons exist without justification? Spell it out coherently.

And this: 'But one cannot rationally believe prisons are a good thing...' Who said? Do you apply that argument to the armed services? Or mental institutions? Sweeping statements won't help you.

OK, how can one rationally believe prisons are a good thing? How can putting a criminal into a place from which he will return with even more psychological problems be a good thing?

About armed forces, well, I think the issue is slightly more complicated. While a military with sane and competent leaders can probably save more people than they kill, that's usually not how it happens.
It's often stated that the involvement of the United States in wars brings people freedom and capitalism. Well, at the beginning of the World War 2, the UK and the USA were giving money to the Chetnik party, which turned out to be cooperating with the Fascists. Chances are, they just made the World War 2 even worse, at least on the territory of the modern-day Croatia.
And I don't have a very high opinion of the Croatian military either. Sure, some military operations of the Croatian War of Independence, such as Bljesak (Flash) and Oluja (Storm), are widely agreed to have been one of the best-organized military operations in modern history. But such things are relatively rare. Sometimes the military leaders are not sane. Slobodan Praljak, for instance, killed many people just because they were of different religion and later killed himself.
Sometimes the military appears to be incompetent, and to lull people into false sense of security. Perhaps the best example of military lulling people into false sense of security is Vukovar Massacre. Basically, a Serbian named Жељко Ражнатовић organized an illegal army to attack a hospital at Vukovar, because it was supposedly filled with people fighting for the independence of Croatia (and therefore deserve to die). The Croatian military was present in the city, and people were certain they were safe from that illegal army. The suggestions about evacuating the hospital were scorned at. However, when the Croatian military leader present in the city, Mile Dedaković, warned of the imminent danger, and the president Franjo Tuđman promised to send him weapons (it is a bit controversial whether he even promised to send him weapons)... those weapons mysteriously disappeared. Needless to say, the illegal army killed many people, and the Croatian military ended up doing nothing but having lulled people into false sense of security. This reads like a poorly-written horror story full of plot holes. But you know as they say, reality is weirder than fiction because fiction at least needs to make sense to the author. Events during the war do not need to make sense to anybody, and the plans made by the government to keep its citizens safe in the case of foreign invasion often turn out to be gibberish. BBC news at the time even suggested the Croatian government let Vukovar Massacre happen on purpose. I do not believe that, but I cannot deny the Croatian government handled it worse than if they did nothing.

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The argument isn't actually about hell or prisons. Flat contends that it's ridiculous that some person was thrown into jail for a tweet.  It is ridiculous, but it did happen.  Guy got processed and paid a fine.  Relatively mundane compared to how it used to be in the region.

That nonsense above is filler.  He knows that Bad Stuff™ also exists.

So, what makes you think that it did happen, that it is not anti-Croatian propaganda? And what do you mean by Relatively mundane compared to how it used to be in the region.?