Are you sure?
Several years ago, I remember having a conversation with a graduate student in philosophy on the subject of certainty. We discussed the subject for over an hour, weaving references and tangents and explications through our conversation, always coming up just short of agreement. Then we realized something: the entire conversation had been based on two different definitions of "certainty" -- he of epistemic, and I of psychological, concepts of certainty.
This experience may be why I feel so strongly about defining terms before discussing major ideas or issues. Yet our confusion illustrates one of the cardinal difficulties in this topic. I was so sure during the conversation that we were discussing the same thing, but I was later to discover that sense of certainty to be misplaced -- that is, totally in error.
When we commonly speak of "certainty," we mean a sense of psychological certainty: the subjective sensation that we have grasped an objective truth.
These sensations may be felt with varying degrees of finality or confidence. We might simply be speaking of fulfilled expectations, the psychical satisfaction of being proven right. "Of course he was cheating on you -- he brought you flowers last week." A stronger certainty may arise when we can think of no refutation at a given moment; stronger still when we can imagine no refutation can exist. The most extreme cases of psychological certainty occur when an idea is perceived as a truth in which no error can ever be found and against which no inconsistent statement can possibly stand.
Sensations of certainty are almost invariably unwarranted.
Humans are, quite simply, human. Our experiences are finite, our perspectives are limited, and our senses may often be misleading. Given the frailty of the human condition, how can we speak of being certain of anything? Moreover, given the hubris to which humans are so often subject, how can we take seriously any statement of absolute certainty?
Here lies the cardinal difficulty: given our own humanity, what can humans possibly know?