The biggest discovery of 19-th century philosophy was due to Kant, who discovered that you can have a framework that is entirely consistent with Plato, Aristotel, Leibniz et al, and at the same time consistent with empiric data, and with itself, provided that you accept that this framework is kinda subjective.

The biggest discovery of the 20-th century, due to Baudrillard, Barthes, et al (pardon my lack of French philosophy background), and at the same time by Wittgenstein, McLuhan and probably many other people, (actually, Hume was there all along) is that a framework that is subjective can actually be consistent with anything you want, as long as you don’t pose any specific criteria to what “subjective” is.

The biggest discovery of the 21-th century, is still due, I guess, but it would probably be some formalization of the idea of the subject, which would be a full circle towards the times before 19-th century.

Meanwhile in the realm of science:

Newton discovers a scientific framework that is entirely consistent with Plato et al, if only you have the concept of an absolute space, but the concept of absolute space doesn’t agree with observation.

Einstein discovers a framework that that doesn’t include the concept of absolute time (which he doesn’t admit is Kantian).

Quantum mechanics discover that a framework that does not rely on the concept of absolute time is inherently subjective (depends on observer).