• If you think about it, so much of life is about chance and what if.

    Like this past weekend, what if that SUV had exploded in New York's Times Square?

    What if that cop wasn't so alert or maybe the driver not so inept?

    What if the "shoe bomber" had gotten his shoe to light or the "undies bomber" his briefs to do the same?

    I suspect history would be very different. Just as it would have been different had a guard been standing outside Lincoln's theater box that night or JFK had used a top on his motorcade that day.

    But none of those things happened because the unexpected did happen. And it got me thinking: History is defined not by the things we do see, but the things we do not.

    We try and learn, but more often than not, we repeat and fail.

    Taking great caution not to ever again launch a space shuttle in frigid conditions, but forgetting about the very real risks of returning one to Earth under more vulnerable conditions.

    Just as we try and stop another 1929 financial crash by preventing the kind of bubble that led to that crash, but never thinking there could be other bubbles leading to other crashes.

    Such is life; such is history.

    Where the one thing you can count on is you can count on nothing, save this: Horrible stuff happens and we count ourselves grateful, as we should, when bad guys are stopped.

    And life goes on, forgetting all the while they have but to succeed once and life as we know it does not. Always forgetting it's the bad stuff — the unexpected stuff — that defines us, not the day-to-day certainties we like to think comfort us.

    Watch Neil Cavuto weekdays at 4 p.m. ET on "Your World with Cavuto" and send your comments to cavuto@foxnews.com