I think Chez summed this one up pretty succinctly by characterizing knowledge as 'the aim of beliefs, that can never be reached'.

Just to expand on that. We approach knowledge and truth by judging the extent to which our beliefs match the outside world, or by predictive value. This isn't just the mantra of epistemologists, it's also the scientific principle. We predict a solar eclipse and one happens at the right time to the second, when the previous one happened decades ago. We can't strictly speaking rule out a fluke, but the chances of a fluke seem so astronomical (no pun intended) that we put faith in the beliefs and understandings we used to calculate that eclipse. Then we do it again, and again, and everytime increase our faith that we've discovered a truth. But no matter how many times we do this, the possibility that we were just lucky still exists, with decreasing odds, so true knowledge can never be verified. IMO that's how knowledge works, and why its at the same time tangible/real and unattainable.

Edit: Should say also that I'd apply the same principle to past events. Only in those circumstances it's memory/perception that are the variables.