Descartes and Kant tried to reach the truth through pure reason — Descartes most famously, wiping the slate clean and trying to rebuild the world from nothing but thought. Everyone takes "I think, therefore I am" as bedrock. But I've always thought there's a crack in it.
That thought could never have occurred to him without a lifetime of interacting with the world. There's no "I" without a universe to stand in relation to. A person born with every sense severed could never run Descartes' experiment — there'd be no concepts to think with. Of course, that objection smuggles in exactly what Descartes refused to assume: that other people and a physical world exist out there. So strictly, all I'm left with is the thoughts I'm having this instant, with no right to assume anything around me. And honestly, it doesn't matter — a convincing simulation of a world is, to my mind, equivalent to a real one.
So I end up believing in an external (or simulated-external) world not because I've proven it, but out of necessity: my mind and my reasoning simply stop working without that assumption, and a couple of others like causality. Which is the real lesson. Pure logic gets you nowhere without a few premises taken on faith. The actual job of philosophy isn't to reason from nothing — it's to figure out the smallest set of premises you have to grant before reason can start. And nearly everyone who's done that honestly seems to end up in the same place: a world that's ordered, external, and understandable through science.