Then why don't you just point to evidence of his existence being more likely than others'? We have bodily remains, intact own writings, or historical records made during the lives of many born in 6th century, e.g. Columbanus, Pope Gregory I, founding emperor of Tang Dynasty, Radegund, Venantius Fortunatus, Theodora). So why don't we have any one of those types of evidence about Muhammad?
Historical Muhammad not certain: http://www.wsj.com/articles/SB122669909279629451 . Of course, people have set about trying to protect minds from a 'fringe' Bayesian view: "Prof. Kalisch was told he could keep his professorship but must stop teaching Islam to future school teachers." In case anyone missed it, Richard Carrier explicitly used Bayes on question of historical Jesus. I don't know if Kalisch used Bayes, but his language conveys intuitive Bayesian update.
The bearing of fictional stories is simple: calculate probabilities of historical X based on practically 100% probability that human imagination was a factor (given that the stories contain highly unlikely magic like in known-to-be fiction stories, plus were written long after X supposedly lived). Note that that still leaves out probabilities of motivations for passing fiction as nonfiction like Joseph Smith or L. Ron Hubbard did. Once you figure probabilities including motivations and iterations of previous religious memes, it becomes increasingly unlikely that X existed. Paul Bunyan, AFAIK, wasn't based on previous memes for controlling people, nor were the stories used to control people, so I wouldn't be suspicious if someone believed the stories started based on someone real. When people insist religious characters were real, OTOH, I become suspicious of their motivations, given unlikelihood that they examined evidence and updated Bayesian-like.
@Salemicus: Citation for "We do ask JK Rowling what non magical boy inspired Harry Potter"?
You left out the 'magical' part of my question. If magical beings exist(ed), then everything becomes more mysterious. That's partly why we don't pester JK Rowling about what extra-special boy Harry Potter was based on. We don't even suspect comic superheros like Batman, who has no magic, to have been based on a real-life billionaire. We certainly don't have scholars wasting time looking for evidence of 'the real Batman.' Modern stories of unlikely events are easily taken as imaginings, yet when people bucket a story as 'old/traditonal', for some people, that bucket includes 'characters must've been real persons', as if humans must've been too stupid to have imagination. https://en.wikipedia.org/wiki/Fakelore
Would you say the origins of other religions become more mysterious if there never were whatever magical beings those religions posit? Would you think it likely that Guanyin was real human of unknown gender? Do the origins of fictional stories become more mysterious if there never were the fictitious characters in the flesh? Did Paul Bunyan exist, as there were similar lumberjacks?
You're not supposed to tie yourself to any hypothesis, even if mainstream, but rather update your probability distributions. Bits of the NT weren't written until long enough after the supposed death of Jesus that people wouldn't have been like, 'Who you talkin' about?' And I doubt they would've cared whether the character existed, like no one cares whether Harry Potter existed, because it's the stories that matter.
elharo was referring to 'veil of ignorance,' a concept like UDT applied by Rawls to policy decision-making.
From what I've learned about brains, the left brain is engaged in symbolic thinking about a problem, which engages more logical, methodical problem-solving. For a combination that you won't arrive at through that approach, you have to give your brain, apparently involving activation of the right dorsolateral prefrontal cortex and other right-brain parts, more time to integrate info from stored memory or lower-level processed stimuli or to make novel associations related to the problem. When left prefrontal cortex is engaged in focusing on performing a task, it'll inhibit the processing of info seemingly irrelevant to the task. This is why aha/eureka moments are more likely when you're relaxed, not focused and your mind gets to wander (e.g. getting on bus while on vacation, taking a shower/bath). Studies suggest that more creative or sudden-insight (as opposed to deliberately trying different combinations) problem-solvers have greater right brain activity and lower inhibition of it.
Look up "Aha! moments" in the index of Eric Kandel's book, The Age of Insight, which cites many papers, incl. "Explaining and inducing savant skills: privileged access to lower level, less-processed information". A few of my other references: "Bayes for Schizophrenics: Reasoning in Delusional Disorders", "Creativity tied to mental illness", "Through the Wormhole: Creativity Cap"
Did you know about Humanity United or other orgs for reducing human trafficking? http://www.forbes.com/sites/clareoconnor/2012/11/08/inside-ebay-billionaire-pierre-omidyars-battle-to-end-human-trafficking/
You're referring to the phrase "many villages were ravaged by the killing of the Arabs of Muhammad", written after Muhammad's supposed death, "Arabs of Muhammad" meaning 'Muslims' the way "people of Christ" means 'Christians'. That Muslims and Christians existed doesn't mean the characters they invoked to justify violence, supremacism, etc. existed as actual humans.
Criteria for considering Muhammad and Jesus near certain are so lax, we'd have to consider some Greek/Roman gods near certain.