"Because murdering Meritxell is a really weird thing to do, and Keltham does a lot of really weird things, while Carissa doesn't do really weird things."

"I think it's Keltham because having a summoned outsider kill someone is the kind of thing you do if you have an instinctive aversion to killing them face to face yourself. which Keltham probably does because he's from dath ilan and which I don't because demons can wear human faces if they care to."

(Keltham doesn't particularly notice that it's Carissa, the other suspect, arguing that Keltham did it; this is obviously meta-Carissa not suspect-Carissa talking.)

"Everyone's normal from their own perspective, and in Civilization, I do rather get the impression, killing people is considered much less normal than it is in Golarion."

"On the other hand, yes, my thought processes are relatively alien to you, so if a weird thing happens, you might assume I was the one more likely to do it."

"And when it comes to murders, if you can avoid killing somebody face-to-face who, you know, is getting resurrected a day later, you'd probably do it that way whether you were Keltham or Carissa, so you're less likely to get caught."

"Let's say for purposes of thought experiment that all those considerations exactly cancel out.  In fact, if we didn't know anything about the argument Ione saw, or know that the murderer could summon a lantern archon, and we just knew that Meritxell was killed by somebody not face-to-face, we would've thought that Keltham and Carissa were exactly equally likely to be the murderer."

"Where do we go from there?"

"....try to get them to confess," says Gregoria after a pause to consider whether they do this in Taldor.

"Surprisingly, whichever of Keltham and Carissa is the murderer does not seem to find it to be the optimal course of action given their own self-interests to politely tell you that they did it."

"I think the criminal justice process in dath ilan must be really different."

"On the one hand, yes, on the other hand, I would've if anything guessed it would be different in the opposite direction.  Is this an Intelligence 10 thing where actually lots of murderers will just say 'Yes' if the police ask them whether they committed the murder?"

"I'm not trained in interrogations but at least in the popular understanding, you tie them to a chair and shake them around a little and tell them you've already got it figured out but it'll go easier for them if they admit it than if they keep denying it, and ask them questions from slightly different angles, and point out contradictions in what they said, and maybe hit them, and they have a hard time thinking about their long term self-interest."

"I suspect this is in fact a different experience at 18 Intelligence and whatever I turn out to have in the way of Wisdom and other stuff your system can't measure at all."

"If you already had it figured out, you wouldn't need to tie me to a chair.  The claim is completely implausible."

"Asking me questions from slightly different angles can be done without tying me to a chair.  Pointing out contradictions in what I said is stupid, you should let me keep talking without telling me how I'm giving myself away, the same way I didn't speak up immediately when I realized the people around me had suddenly acquired Arcane Sight, just in case that was being hidden for some actually interesting reason."

"Hitting me does not seem particularly likely to shift how much I consider my long-term interest, except insofar as I decide that I clearly have a very strong long-term interest in certain parts of Governance ceasing to exist."

"I hope I don't need to point out that if we're considering applying enough pain, matched to a light enough penalty for the actual crime, that somebody who yields to threats, thinks it's in their direct interest to confess, this is exactly as likely to work on innocent people as guilty people?  Unless the true murderer is known in advance to have information they can give up, such that the police can verify with near-certainty that the confession was true?  Otherwise it's isomorphic to offering somebody a thousand gold pieces to confess to a crime whose penalty is a hundred gold-piece fine.  Sure, the guilty party will confess, and so will any innocent ones.  You're putting them in a situation which isn't a Lawful scoring rule, you're not offering maximum reward for true confessions distinct from false confessions and you shouldn't expect the words people say to communicate anything."

"Unless you think you're supposed to tell the truth even when somebody has tied you to a chair and is hitting you?  In Civilization it would just be considered obvious that this is not a cooperative setup where you have an interest in your words being heard as meaning things, you mostly want for the situation you're in to not exist in the first place, so everyone will just try to trash the setup as hard as possible."

"- again, not trained in interrogations, but I think people do lie, but the lies you tell if you're innocent and the lies you tell if you're guilty are not the same. The details you make up, stuff like that." Carissa doesn't know, maybe interrogation in Taldor is in fact totally ineffective, it wouldn't be very surprising.

"Yes!  It's the same way in Civilization!  Innocent people don't always talk exactly the same way as guilty ones!  That is in fact why police in Civilization talk to suspects in crimes even if people won't just politely tell you they did it!  Except that in Civilization the police don't hit all suspects including the innocent ones, thereby maintaining a cooperative stance with the innocent parties but not the guilty one, which incentivizes everyone except the actual guilty party to give the police as much actually-true information as possible and forms a further factor distinguishing the guilty party's behavior which helps the police distinguish them."

"Maybe that works better? I'm not strongly committed to our system working better. But that's how it's usually done, here. ...not if one of the two of us murdered Meritxell because this is an important government project and they would try harder than usual."

"Okay, you know what, I'm going to say this even knowing exactly how much I'm tempting the 'tropes':  If there is an actual criminal mystery around here, I am going to take charge of the investigation, and nobody is to do any hitting of anybody while we try it the dath ilani way first."

"I'll pass that along," says a Security from the doorway, with no noticeable expression or emotion.

Internally, of course, he is ranking all the girls by how much he wants to make sure not to miss their executions when this whole thing blows up.

"Probability," says Ione.

"Right, so, the general direction I was aiming for, here, was people starting to talk about how likely it would be that Carissa versus Keltham would be able to summon a lantern archon, or how much more likely Carissa is to get into an argument with Meritxell in worlds where Carissa committed the murder versus worlds where Carissa was innocent."

"It's possible, I am realizing, that this is less of an obvious next question if you haven't grown up hearing adults talking that way all the time."

From the confused faces looking up at him, yeah, seems non-obvious. 

"Do my eyes deceive me?  Are the people who are actually confused, looking visibly confused enough that I, their teacher, have any idea of what's going on inside their minds?  Thank you.  It significantly helps and I appreciate it.  Now, those of you not looking confused, is that because you totally understood where I was going there, or because you forgot to violate your usual habits and look confused on purpose?"

"It's the second one.  This is going to take some effort, actually."

Gregoria is looking confused on purpose. It is kind of terrifying but that's life on Project Lawful for you. "I don't - see how Carissa and Meritxell having an argument is relevant to the probabilities at all. And I don't see how you could possibly get sure enough for it to make sense to do anything other than, uh, expensive fancy Truth Spells or whatever the Crown keeps in store."

"The problem I'm having with thinking up simpler and more realistic examples is that I haven't been in Golarion long enough to learn a lot of probabilities and use them myself on problems with this structure.  The cases where I've resorted to explicit reasoning of this form, have been about weird exotic things I couldn't figure out by simpler methods, like the timing of the Zon-Kuthon attack relative to when I went outside the Forbiddance, or the implications if Carissa mysteriously failed to make her afterlife arrangements."

"Okay, uh, hopefully much simpler example.  Around what fraction of wizard students in your academy had Intelligence 16, 17, 18?"

"....most are fourteen, actually. Most top students are sixteen, seventeen is rare and eighteen's even rarer."