One of my "favorites" is the argument that nobody can really know anything, perhaps due to the problem of induction, perhaps because science cannot prove that the scientific method works, or because every belief is just arbitrary opinion or ideology anyway, even science. This is obviously what comes out of the south end of a north facing bull because the people who argue like that still leave their houses through the door instead of attempting to walk through walls, they still board airplanes, and they still trust that their jacket will not spontaneously turn into a tree while they are wearing it. In other words, they do believe that one can tentatively gain knowledge through observation and the use of reason and rely on this knowledge, they only pretend not to do so when shown that another of their beliefs (e.g. in some supernatural process or entity, or in the efficacy of some "alternative" medicine) is ludicrous.
Here is another one that I have recently come across:
Absence of evidence is not evidence of absence.Sounds reasonable, right? At least at first sight, and from the perspective of an armchair philosopher. But do you actually reason like that, as we say these days, in real life? What would you say if you met a well-educated adult who argued, well, I have no evidence that smurfs exist, but I cannot disprove their existence either, so they may well exist? Would you consider that reasonable? No, you would rightly consider that person to be an idiot, or at a minimum to be somewhat unsound of mind.
Of course where I did encounter the argument was once more in the context of discussing certain supernatural entities, where for some bizarre reason people seem to consider it legitimate. But it could just as well be used to defend claims for the existence of alien visitors, Bigfoot, unicorns, the continent of Lemuria, reptilian shape-shifters from the hollow center of the planet, and other spurious nonsense. And that is just the point.
So what this sentence should look like is this:
Absence of evidence where we would reasonably expect to find evidence is tentative evidence of absence.Does not roll off the tongue quite so easily but it has the advantage of not being as stupid.