Certainty is a state of mind. One who is certain is one who does not doubt that some X is true. Having certainty regarding X does not guarantee that X is true, but merely that one believes X is true and has no doubts regarding its truth. Someone who seeks certainty regarding some X, then, seeks to justify belief in X to such a degree that they no longer have doubts regarding the truth of X.
Many post-modern types decry the desire for certainty as an “Enlightenment ideal,” preferring questioning and doubt instead. This is wrong. The desire for certainty is a basic human desire that has manifested itself in every generation. Humans want to know that what they believe is true. While certainty is not required to have knowledge (and philosophically speaking, not possible for most things), and while certainty is not required for everything we believe, and while an inordinate desire for certainty can be bad, the desire for certainty is natural, good, and obtainable in some matters.
Thinking to Believe 