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Explanation
This comic satirizes the tendency of philosophy students (or philosophically-minded people) to derail serious academic discussions with radical skepticism. A presenter is about to give a talk on the nature of justice in relation to the carceral state -- a substantive, real-world topic -- when an audience member asks them to first prove that everyone in the room actually exists. This is a reference to solipsism and Cartesian doubt, the philosophical position that one cannot be certain of anything beyond one's own mind.
The humor lies in the absurd mismatch between the practical, policy-relevant subject of the presentation and the audience member's demand for a proof of basic external reality. It's a common frustration in philosophy: no matter how important or applied the topic, someone can always regress to more fundamental epistemological or metaphysical questions.
The caption -- "Sometimes I wish I were a philosophy professor, just for the oral exams" -- adds a second layer, imagining how entertaining it would be to watch students try to navigate these kinds of infinite-regress traps in an exam setting.