Prisoners chained up in Plato's cave develop a self-correcting system for cutting and controlling the shadows on the wall. Their science is validated by increasingly consistant predictive success. Of course, one of the prisoners gets out of the cave for a bit. She walks on the grass under the sky, feels the breeze on her cheek, soaks up the sun's warmth, takes in a multitude of unfamiliar sights, scents, sounds, tactile and proprioceptive sensations and struggles with focal selection.
Of course, she is returned to her place in the cave. At first, because her eyes are adapted are to light levels higher than she'd ever experienced in the cave, she is unable to see the shadows on the wall, and when her eyes do re-adjust, she is unable to treat the shadow games with the gravity that her fellow prisoners do. She tries to explain what she has experienced, but she has only the vocabulary developed to describe the shadows on the wall with which to try to convey to her fellow cave-dwellers what she experienced outside of the cave.
We all know this story, so I'll cut to the chase. One of the prisoners is insistent that the rationality of the cave bodes ill for the notion that there is an "outside." Cave Rationality (CR) is pretty labirythine stuff, so it's hard to be sure, but none of the big shadows in view just now lend any credence to this whole "outside" business, and more to the point, he doesn't like being told that the CR at which he is impressively adept is somehow inferior to this fuzzy "outside" bullshit.
Because she can't present any sort of convincing case for her experience using CR (she doesn't even try--the absurdity of such a project prompts lingering bouts of maniacal laughter) the CR silverbacks label her continued belief, which flies in the face of all available evidence (or seems to at first blush--again, this CR stuff is pretty complex, and nobody can credibly claim to have followed its every twisting corridor to its final conclusion), faith. And what is faith? Faith is exempting in principle some representation(s) from rational criticism.
Now, one of the prisoners who is situated close to her, and who was never one of the CR superstars detects an unfamiliar scent on the prisoner who claims to have been outside the cave, and what's more, her story just seems to "ring true" to his ears, and the unfamiliar scent, for him, adds credence to her story. This prisoner is now a believer. The silverbacks don't smell anything out of the ordinary, of course, and what's more, CR doesn't recognize "strange smells" as legitimate evidence.
The silverbacks roll their eyes dismisivley at the new believer. She has infected the believer with her faith. The silverbacks call the believer's acceptance of her story "faith," and they apply this same lable to her belief that her experiences were something other than a hallucination, but wouldn't you agree that his belief and hers are rather different? They can be expressed in CR semantics with the same string of representations, but they arrise from very different sets of experiences.
Suppose the believer, while only marginally proficient with CR, is charismatic and a good speaker and he manages to convice a few other cave dwellers to share his belief. They believe on the basis of his convincing rhetoric. They did not catch a whiff of the unfamiliar oder that won over the first believer, and so their belief has a different causal history than that of both the one-time escapee and her charasmatic convert. Still their belief can be described in CR lingo as being identical to both that of the one who'd actually been outside and that of her first convert. It's all "faith" to the CR silverbacks.
People come to form their seemingly irrational beliefs in a variety of ways, and to ignore the causal history of these beliefs; to catagorize and define them strictly in terms of their semantic content; to say faith equals x is to opperate with a very blunt instrument.
-KMO