In his magisterial Augustine of Hippo, Peter Brown writes of Augustine, "He wanted complete certainty on ultimate questions." (1st ed., p. 88) If you don't thrill to that line, you are no philosopher. Compare Edmund Husserl: "Ohne Gewissheit kann ich eben nicht leben." "I just can't live without certainty." Yet he managed to live for years after penning that line, and presumably without certainty.
The trick is to tolerate uncertainty without becoming either a skeptic or a dogmatist.
There is a difference between subjective and objective certainty. If subject S is subjectively certain that proposition p, it does not follow that p is true. That would follow only if S were objectively certain that p. But objective certainty appears attainable only with respect to one's own mental states. I am both subjectively and objectively certain that I have a headache now. This is because the esse of the headache = its percipi. Its being is its being perceived. It therefore cannot be intelligibly supposed that I merely seem to have a headache now, while in reality I do not. With respect to a physical object or state, however, appearance and reality can come apart, and what is subjectively certain can turn out to be false: my seeming to see a mountain is no guarantee that there is a mountain. My seeming to feel elated, however, just is my being elated.
What about states of affairs that involve neither mental data nor physical objects? If S is subjectively certain that torture is always morally impermissible, and T is subjectively certain that torture is sometimes morally permissible, then one of the two must be wrong, which shows that subjective certainty is no proof of objective certainty.
What about this last proposition however, namely, the proposition that apart from mental states, subjective certainty does not entail objective certainty? Is its truth merely subjectively certain, or is it also objectively certain? It is objectively certain. One sees that subjective certainty can exist without objective certainty from the fact that two subjects, S and T, can be subjectively certain of contradictory propositions. Here the mind grasps a truth about a state of affairs transcendent of one's mental state and does so with objective certainty.
I conclude that there are some propositions the truth of which can be grasped with objective certitude even though these propositions are not about such mental data as pleasures and pains. The mind has the power to transcend its own states and not only to know, but to know with objective certitude, truths whose truth is independent of mind. That is amazing.
One some days, existence strikes me as the deepest and most fascinating of philosophical topics. On other days, I give the palm to time: "What, then, is time? If no one asks me, I know; if I want to explain it to someone who does ask me, I do not know." (Augustine, Confessions, Bk. 11, Ch. 14)But today, the honor goes to knowledge.