It doesn't seem like there's any good way to handle armchair "philosophers" who think they have something deep to say about the idea of certainty. "See, the thing is, you can't actually know anything for sure! I mean, like, what if we're all just brains in vats?" The only compelling argument I see for this point proceeds by example: The claim is that nobody can truly know anything whatsoever. Anybody who can in all sincerity make this claim must know very little -- perhaps little enough to serve as supporting evidence.
Joking aside, this is one of the silliest ideas out there. Those who contract it generally do so within a year of their first serious exposure to the great philosophers, in that honeymoon phase where everything makes sense and the ideas of "discarding all assumptions" and "questioning everything" still seem novel. Socrates and Descartes are particularly severe offenders, as brief or unconsidered treatments of their works can easily give the impression that they advocated this total uncertainty. What, more precisely, is this misconstrued idea, though?
It's the idea that we just can't know anything for real, man. You know? Like, think about it -- everything we know comes from observing the world, via our senses, right, and how can we trust our senses? What if we all see, like, different colors? What if two plus two doesn't always equal four? What if it could equal five, but we don't believe that because we've never seen it? Or because we can't see it? Would we even know how to count if we were all blind? I, like... I dunno, dude. I dunno. Whoa. We don't even know that. We don't know anything. Not really.
The fallacy in this (admittedly somewhat parodied) line of reasoning lies in an overly strict definition of "know". To "know" something, in this sense, is to know it unquestionably -- for it to be beyond doubt. Presumably, anything which is "known" would, if it could be communicated, instantly and irreversibly persuade any audience of its truth. This is a ridiculously strong sense of the word "know" which, as far as I can tell, is never used outside of this particular context. That is to say, I'm fairly sure this definition is only ever employed by novice "philosophers" for the purposes of making this exact point.
It's tricky to get a strict, rigorous definition of the term out of these people -- they never seem to quite know what they mean by it. Go figure! Usually, though, if they're pressed enough, words and phrases like "undeniable" or "beyond doubt" or "with certainty" start getting thrown around. "100%", maybe -- as in, "to 'know' something is: to be 100% certain that it's undeniably true", or some such schlock.
The argument, then, always goes, "how do we acquire knowledge? Hah, trick question, we don't! We can't rely on our (senses|reason|memory|etc), so everything we could ever use to learn is flawed! And if it's all flawed, then we can never truly know anything!" This is followed by either a look of wonder or smugness, depending on the person in question. An alternative argument is a bastardization of the classic Socratic argument, which concludes that the only true knowledge to be found is the knowledge that you know nothing -- aside, of course, from this knowledge-of-ignorance, which you just proved you do know -- don't worry about it. If these arguments were rigorous then we'd be mathematicians.
The story behind this one, as I've heard it, goes that the Oracle of Dephi was asked who the wisest man in Athens was, and she replied that Socrates was wisest. Socrates was like "wat" and took it upon himself to prove this wrong. To do this, he started seeking out all the specialists who he'd expect to have any knowledge, and started questioning them about their areas of expertise. As the Dialogues show, none of them held up particularly well. Socrates eventually concluded that all these people knew no more than he did, but that they fooled themselves into thinking they knew more than him, and so in a weird way this self-deception meant that they knew less. Socrates, after all, at least was honest with himself about how little he knew -- if nothing else, he knew about his own ignorance. Half-comprehending plebs will find a thousand ways to twist this around into some sort of ultimately agnostic statement about knowledge being impossible.
The main flaw with those arguments, -- a flaw which gets overlooked surprisingly often -- is that they literally take the word of a fucking oracle as one of their unquestioned premises. Fun fact: Modern geologists are pretty sure that those oracles were just high as shit all the time, likely thanks to their (conjectured) proximity to natural flows of ethelyne. This could be seen as a minor objection.
Where does this extreme definition of knowledge even come from? I don't know, but I wish it would go back there. Who actually uses this meaning day-to-day? "Hey, do you know what time it is?" is just a request for the time of day, ± a few minutes. You don't need to know & communicate some sort of deep, transcendental knowledge (whatever that would even be) in order to have a conversation.
The tragedy here is that we have so many good ideas for what "knowing" could be taken to mean. In the philosophy of science, it is typically postulated that, while we can never "know" with certainty, we can consider something an evidence-backed theory, and therefore "known", if lots of people have tried hard to prove it false and failed. For another definition, the phrase "justified true belief" manages to get thrown around in philosophy classes from time to time, despite its being wholly silly. And, of course, most people day-to-day don't use either of these rigorous definitions when they're using the word "know" -- they use something closer to the dictionary definition, "to perceive directly". By this sense, what you know is some sort of informal aggregation of the things you've perceived. And, if none of these sit right, you can fall back on the fact that nobody's actually unsure what this word means, because come on: when it comes down to it, you know what you know.
There is a certain category of cliches to which this "we can't ever know anything for sure" idea belongs, along with the classic "don't trust anyone -- not even (me|(this (book|blog|article))!" and other such sillinesses. These are tropes which might sound cool or insightful the first few times you hear them, but which radically oversimplify things and quickly get worn out. Such cliches are all but useless -- at best, they only manage to gesture broadly in the direction of something worth expressing. Somehow, a lot of people seem to be content with that. I just wonder, though: Can't we, you know, do better?