I've been away! Have I been away? Or have I just been dreaming or hallucinating?
There was a a chap in the seventeenth century, René Descartes, who opted for reason over a divine source of knowledge. He probably wishes he'd never said, "Cogito ergo sum" ("I think, therefore I am"), because it's all anybody remembers about him.
He arrived at the cogito through an experiment in radical doubt to discover if there was anything he could be certain of; that is, anything that he could not doubt away.
He started out by doubting the existence of the external world. That was easy enough. I do it all the time! Then he tried doubting his own existence. But doubt as he would, he kept coming up against the fact that there was a doubter. Must be himself! He could not doubt his own doubting.
Maybe he should have said, "Dubito ergo sum."
There can be no doubt that I've been to Bali! I've got the sunburn to prove it. And the prickly heatrash in the groin. And an empty wallet.
"I'm sunburnt and heatrashed and without money, therefore I must have been to Bali."