monism
Explanation
The Joke
A professor is lecturing about monism -- the philosophical position that mind and body are not separate but are aspects of the same underlying reality. He explains that dualists believe mind and body are separate, while monists hold there is no fundamental distinction. He then notes there is a "lively debate" but that the important thing is that both sides are "talking about the same damn beings" -- hinting that the debate itself is somewhat circular.
By induction, he extends this logic to argue that you can be a "monist dualist" because the debate between monists and dualists is really about the same subject, and therefore they are "literally the same." He claims this makes him a "ten-to-the-twenty-seventh-power-ist" because he believes every one of the distinctions people draw about reality is "meaningfully vacuous." A student asks if he has reduced philosophy to a "vacuous work on free will," to which the professor responds "you may also be right."
The Humor
The comic satirizes academic philosophy by taking the core claim of monism -- that seemingly distinct things are really one -- and applying it recursively to the philosophical debate itself. If monists say everything is one, then surely monism and dualism are also one, and the distinction between those positions collapses. The joke escalates this into absurdity, with the professor proudly declaring himself a believer in an astronomically large number of collapsed distinctions, effectively claiming that all philosophical positions are the same. It is a playful reductio ad absurdum of monist reasoning, and a gentle jab at how philosophy can sometimes disappear up its own abstractions.
References
Monism and dualism are longstanding positions in the philosophy of mind. Dualism, associated with Descartes, holds that mind and body are fundamentally different substances. Monism, in its various forms (physicalism, idealism, neutral monism), denies this distinction. The comic is riffing on the tension inherent in trying to argue that "everything is one" -- a claim that, taken to its logical extreme, undermines even the distinction between itself and its rivals.