vat
Explanation
This comic is a riff on philosophical skepticism about external reality -- specifically the "brain in a vat" thought experiment.
In the first panel, one character challenges another by pointing out that we can never truly know whether we are physical beings experiencing reality or just brains in vats being fed simulated experiences. He asks: "Isn't it at least POSSIBLE that you're in an actual body experiencing actual reality?" This is a clever inversion of the classic thought experiment, which normally asks whether it's possible we are brains in vats. Here, the character is walking through a beautiful park and treating the brain-in-a-vat scenario as the default assumption, so the radical skeptical position has been flipped.
The second character dismisses this, retorting: "You're going to posit an entire cosmos just to give me the experience of walking through a park this morning? Be reasonable." This is funny because the character treats the idea that actual physical reality exists as the extravagant, unreasonable hypothesis, while assuming that being a brain in a vat receiving simulated inputs is the simpler explanation.
The caption at the bottom reads: "Technically, external-world skepticism is the most parsimonious theory." This references Occam's Razor (the principle of parsimony), which suggests we should prefer the simplest explanation. The joke plays on the fact that, in a narrow sense, positing that only your own mind exists (solipsism) or that you are a brain in a vat requires fewer ontological commitments than positing an entire external universe. Of course, virtually no philosopher actually endorses this reasoning in practice, which is what makes the comic's deadpan delivery so amusing.
The comic engages with ideas from Descartes' "evil demon" scenario, Hilary Putnam's brain-in-a-vat thought experiment, and the general problem of skepticism about the external world that has occupied epistemologists for centuries.