What's a proof?

What constitutes a proof? Does establishing cause for an effect constitute adequate proof? Or is the whole notion of proof merely a matter of perception?

Clarity

I believe troubled times are actually the only moments in life when we see things for what they are worth; that we give value to things since they don't have an intrinsic value themselves. We claim that some things are more valuable than others. In fact, so successfully have we conditioned ourselves to believe it that we're unable to break out of it and cling on to our "values". Even when the thing that we value has lost its meaning to us, we cling to the past, unable to let go.

Thank God for troubled times!