Nihilism
Explanation
The Joke
A nihilist proclaims that nothing matters and life is meaningless. Another character agrees — and then asks the nihilist why they bothered getting dressed, eating breakfast, and coming to this conversation if nothing matters. The nihilist has no good answer.
The Humor
The comic points out that true nihilism is self-defeating in practice. If nothing matters, then the claim "nothing matters" also doesn't matter, and there's no reason to advocate for nihilism. The fact that nihilists continue to eat, sleep, and argue about philosophy demonstrates that they don't actually believe their own position at a behavioral level.
Philosophical Context
This is close to the existentialist response to nihilism: that the absence of inherent meaning doesn't free you from the need to act. Camus wrote that "one must imagine Sisyphus happy" — meaning that even in the face of absurdity, we continue to engage with life. The comic's nihilist is a Sisyphus who insists the boulder doesn't exist while continuing to push it uphill.