Answer by Jim Ashby:
Yes, in the strictest sense, one can't absolutely prove anything. Everything is built on assumptions. However, I think René Descartes had it right: 'Cogito ergo sum' (I think, therefore I am). If you're thinking, you must exist. Consciousness is the most irreducible 'fact' we know of.
But we don't typically indulge epistemological and ontological circle-jerking in daily life: that's a good way to paralyze thinking altogether. Rather, we accept that our substrate of assumptions work pretty well. I assume that what is red to me is red to you too (if you're not color-blind): that redness is objective, not subjective. In practice we normally regard evidence, proof, facts, reality and truth, to be those things which are 'confirmed to such a degree that it would be perverse to withhold provisional assent' (nod to Stephen Jay Gould). That's the litmus test I use to declare something 'objective': objective reason, objective fact, objective support, etc.
I know it's discomforting to accept that perceived reality is an illusion but it should help to know that it's not a wild illusion. Human perception and interpretation of objective reality is pretty damn reliable despite the limited scope of our senses. Our understanding is 'close enough' to spectacularly advance science and technology. The regularities in nature that we call 'the laws of physics' are consistent and persistent enough to take men to the moon and back (and scramble to solve unanticipated emergencies), safe and sound. It's consistent and persistent enough to allow the functioning of this thing called the Internet . . . and rovers on Mars . . . and probes in the Oort cloud. I call the level of understanding necessary to pull these things off pretty damn reliable.
People throughout history have experienced the foreboding, awesome, transcendent, grandeur of spiritual experiences. I know I have. Maybe you have too. In the past, this subjective phenomenon has been one of the strongest personal arguments for the numinous: not because the experience is so powerful but, rather, because so many people experience it. There's something definitely going on.
But now we have growing evidence that it's actually just a neurological phenomenon. To me, phenomenology is not a problem for atheism. It's just a post-modernist spin on apologetics.
Phenomenology has been separately developed in philosophy, psychology and religion. I'm addressing only the phenomenology of religion.