I just want to pipe in to note that the notion of certitude is a bit fuzzy. If something is sufficiently probable, depending on the context, we just call it “certain”.

I mean, there is my certitude that 2+2=4; it is really hard for me to imagine any circumstances in which I could be wrong about that, although I suppose if I am insane I might imagine basic mathematical truths that aren’t actually true at all.

Then there is, say, the certitute the interior angles of a triangle sum to two right angles; which people took as certain until we discovered non-euclidean geometry, which turned out to give us exceptions we hadn’t previously considered.

Then there is my certitude that my wife is upstairs right now, or my certitude that a man named Augustus ruled Rome. could I be wrong? I suppose it is theoretically possible (maybe my wife was abducted by aliens in the last 10 minutes, maybe all the testimonies to the existence of the Emperor Augustus were medieval forgeries), but (all things considered) such mistakes seems such remote possibilities that I can just ignore them and take those facts as established beyond any reasonable doubt.

I guess the point is that a radical skeptic can, reasonably, raise a doubt about anything regardless of how well-established it is. Certainty must admit of degrees or there is no certainty.