one could argue that believe in something without a shred of evidence is the ultimate irrational belief, thus rendering all believers in religion clinically "nuts."
(I make this statement with tongue firmly planted in cheek.)
I make that statement with complete seriousness.
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The thing is, the believer may have their own evidence. It may not be convincing or acceptable to anyone else, but it's still rational for them to take it seriously.
Let's say that the FSM actually appeared to you while you were alone in the woods, taking you for a wild noodly ride through the stars before depositing you back where you started. Naturally, nobody else would believe your story... but as far as you can tell, it happened to you. You sensed it. It is completely rational for you to believe your senses. You'd also be rational to doubt them, but it would be
irrational to discount them completely out of hand. Especially if it happened again, or you got a letter the next day saying "Thanks for the fun trip. Peace! -FSM".
My point is that not all evidence is empirical or meaningful beyond the person who experienced it, and that it is perfectly rational for people to believe in that kind of evidence too... even if, in fact, the whole FSM experience was actually the result of someone secretly putting special mushrooms in your breakfast omelet, or even if you are actually just nuttier than a constipated squirrel.
The point is, rational and crazy are not opposites, or even mutually exclusive.