How does one come to certainty about whether a given device is reliable, versus imagining it is? Is faith involved? If so, wouldn't any device sufficiently believed in work/"work"?
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I think you're poking the wrong concepts here, we are talking about divination, an inherently subjective and unreliable practice.
But to answer your question, yes, I would think faith is involved. It is likely that "any" device might work. Usually, I think tradition is involved: The I Ching enjoys some 3000 years of tradition to "back it up", tarot is a lot younger, but still draws on an older tradition: that of the qabalah.
Tarot cards are a bit representative, though: they are "just" a pack of playing cards.
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Even without all the mystical stuff and significance of divination systems, I like them as a sort of book or maybe an algorithm if you will, that will produce new stories each time, so that it is inexhaustible!