dualism
Explanation
This comic jokes about mind-body dualism, the philosophical position (most associated with Descartes) that the mind and body are fundamentally different substances. Two people are discussing: one asks, "Are you a dualist? Do you believe mind and body have a fundamentally different nature?" The other replies affirmatively, saying "as long as we're being introduced, we need some distinction between the two."
The twist comes when the first person pushes back: "You've got mind, body, soul -- why are so big each gets its own category?" The other responds with something about "my mysterious ways." The final punchline is essentially that the person treats their mind, body, and soul as completely separate things -- not out of philosophical conviction but because each one is so large it needs its own classification.
The humor lies in deflating a grand philosophical question (the nature of consciousness and its relationship to the physical body) into a mundane observation about categorization, and poking fun at how people invoke philosophical concepts without really engaging with them.