As regular readers of the comic should know, Gottfried Leibniz solved metaphysics once and for all with his seminal work The Monadology. He posited that causation was an illusion, and that reality was at its core entirely made of Monads, which were eternal, non-causal entities. The only reason that our mental perceptions and physical objects seems to interact is because God pre-established them to be in harmony. So basically, when you raise your arm, it appears that your ideas cause your arm to raise, but really it's only a sort of coincidence, as God set it up ahead of time to coincide.

Zeno of Elea was a pre-Socratic philosopher who us most famous for his thought experiments which proved that motion was impossible. Philosophers since have halfway proved his thought experiments were wrong, but have never quite proved it all the way.

Wittgenstein was a 20th century philosopher, best known for his philosophy of language, where he proved that language doesn't directly correspond to reality, but is an entirely social phenomenon. He did this through a series of about a thousand bizarre, eclectic thought experiments.

Socrates was super annoying, and questioned everything.

Monty Python and the Quest for the Holy Grail was a British comedy, in which many silly things occured.