Most people are uncomfortable with the idea of absolute certainty. No matter how firmly any fact can be established, at the back of their mind they will always think "There is nothing of which I am so certain that I am unwilling to consider the possibility of my being wrong. There have been times when I've been wrong when I felt certain that I was correct, and I'm not smart enough to think of all the possible counterarguments to any position, so how can I know for sure that none of them have validity? The very fact that I don't know whether I can know anything for sure is surely proof of this".
In fact, there is something about which one can be certain, namely truth itself. One is never going to be put into a situation of being rationally forced to admit that truth is not true, for what would this even mean? In order for it to be untrue, there must be some "truth" to which it does not correspond. But truth is just defined as whatever that thing is. That is to say, the definition of truth is so general that its interpretation can always be changed to accommodate whatever "truth" it is claimed to lack. In fact, the very concept of "uncertainty" requires that we be certain of at least truth itself, because understanding truth and accepting it as absolute cannot be separated. Without the concept of truth, "uncertainty" cannot be meaningfully interpreted.
Therefore, one can state without fear that there is a basis for absolute certainty, and that the basis is truth itself. A denial that there is such a concept as "truth" that makes no reference to the concept (which would be to admit its existence) has no relevance to truth, and even if true proves nothing about it. Therefore, the only way to deny that there is a concept of "truth" is to deny that there are any concepts at all. This runs into the same problem as denying that among the possible concepts is the concept of "truth", for now the concept of "concept" must be referred to in the course of declaring it meaningless, and the impossibility of doing so has the same implications for "concept" as for "truth".
It should come as no surprise to anyone that at the end of the day, it is true to say that truth is true according to a true definition of truth. Anyone who denies it and expects to occupy the logical high-ground is a fool, and their denial can be explained only in terms of a flawed human psychology. Skepticism should be viewed as a psychological phenomenon driven by human emotions.