The title *is* in fact whacked out in a sense. I would say that our practical knowledge of the world is certainly valid in the inferential sense. Anyone who enjoys air conditioning, automobiles, modern medicine, and supermarkets has empirical science to thank. But the premise is not totally illogical. When the Buddhists and others say that you should unlearn everything, they don't mean you should un-learn all the math and science and history you took in school.
What you do need to realize, though, is that none of it has *ultimate* validity because *all* propositional knowledge includes, either explicitly or implicitly, a reliance on unprovable first principles. For example, most scientists (and people in general) believe in matter that exists independent of any cognizing mind. This is entirely unprovable (just look at the flimsiness of G.E. Moore's "proof" of the existence of an external world, the best one we have yet in Western philosophy) and is an article of faith as much as believing in God (and George Berkeley notably thought that the latter was easier to believe in than the former). All the amazing progress science has made notwithstanding, many people are prone to the IMO arrogant assumption that these is ultimate truth in those findings. There is not. All scientific, mathematical, logical, and philosophical knowledge (hell, even our common sense) relies on unprovable axioms that employ undefinable terms. We make them up ad-hoc basically to ensure that we have grounding in our daily life and in our research. Evolution has shaped us to see things in certain ways and take certain first principles more readily than others, but then again Evolution also saw fit to bless us with appendixes, which are worse than useless since the only interesting thing they can ever do is rupture and kill you. So do understand that if you claim to "know" anything expressible in language (yes, even "I am"), you are necessarily going to have unprovable presuppositions you rest your knowledge on. These can be more (belief in a vengeful paternalistic sky-god) or less (belief that any two lines on a plane must either intersect or be parallel) toxic. And the only way to de-toxify them is to see that they are baseless assertions that we use because they are convenient and we want to create a body of theory on that basis (including naive realism, which most people don't call a theory at all - but it is).
The thing is, knowing this about our axioms doesn't mean our knowledge goes away. We just see it creatively. The non-Euclidean geometry that Riemann invented, making possible Einstein's work in relativity, came about because he questioned the axioms and undefined terms of Euclidean geometry. He realized that you could still get a meaningful and highly useful geometric theory by tweaking the old Euclidean axioms a little bit, a move many mathematicians saw as rather insane. Truly creative, truly powerful thought is barred to those who treat their axioms and undefined terms as sacred cows, or who haven't yet come to the understanding that unprovable first principles are at the root of everything they call knowledge. So by casting all into doubt, you don't wipe the slate clean. You erase all the useless scribbles, compactify and miniaturize what was on there already, and free up tons of new, empty space upon which you can write new things. In doing this we do, in fact, build on the foundations of others, the way Riemann built on Euclid. We just get better at pruning away the bullshit that comes from seeing axioms as rigidly defined truths delivered unto us from on high.
P.S. If you really want to bake your noodle, consider the fact that "I exist" is an unprovable first principle just like any other. What exactly do you mean by those two words? If you can't exactly say what they mean, then the statement is just a baseless assertion that you take as a first principle.