Thunderstorm Cave

Terry Pratchett's metaphor for how the earliest human beings came round to visualising the notion of Gods.

Imagine a Neolithic person running into a nearby dry cave to keep dry, and escape the worst of the wind and the rain, and with nothing to do except watch the son et lumiere.

If that great human characteristic - imagination - had sufficiently evolved, our Neolith would be sitting there, huddled in his animal skins, feeling thankful that at least the rain was keeping down the sabre-toothed tigers.... and then imagination might kick in, and he would think "What if they've found a nice dry cave to shelter in... like this one?"

With a vague feeling that something was missing from that line of thought, but he couldn't for the life of him think what, our neolith might try to banish thoughts with teeth and growls in them from his mind. He might watch the lighning and think "Where does that come from?" or listen to the thunder and think "That's just like the time Ug and me rolled those stones away to enlarge the cave, only it's noisier... who's out there rolling VERY big stones? A very big Human Being? Is He also doing the lightning thing? If He is human, will he protect this little human from any sabre-tooth tigers? Oh Great Human, don't let there be a bear or a wolf or a tiger in this cave with me..." and thus is a New Idea born , that of religion, based on a kind of logical reasoning:

"All that noise and light was happening up above. Therefore up in the sky there is a place where people live. Do we get to go there when we... leak red stuff and stop moving, like when that bear got Ug before he could stab it? What can we do to make it certain we will go there when we stop moving, leakage of red stuff optional?"

And so on, to the emergence of the first Priests.

Terry Pratchett expands on this idea in the Foreword to the Ultimate Encyclopaedia of Fantasy (the definitive illustrated guide) pub. Carlton books 1998.