tickle
Explanation
This comic takes the familiar observation that you cannot tickle yourself and pushes it into a rigorous philosophical thought experiment. The first speaker notes that it is impossible to tickle yourself, which means tickling is based on the element of surprise. Another character observes that this proves "the discontinuity of selfhood" -- if you knew the tickle was coming even 5 seconds ahead, because you could see your friend's hand reaching toward you, you would still be tickled. So the inability to self-tickle is not just about predicting the sensation.
The discussion deepens: if knowing a tickle is coming in 5 seconds does not prevent it, then the "being" that is being tickled is essentially "not you" in some meaningful sense. The tickle response treats your body as if it belongs to someone else. One character concludes that if we accept you cannot tickle yourself, it follows that the being tickled into existence by another person's touch is fundamentally not the same entity as "you."
In the final panel, someone in silhouette is recounting this argument to their partner, and the partner (seemingly a baby or the concept of tickling itself) just makes "coochie-coochie-coo" sounds, completely ignoring the philosophical ramifications.
The humor lies in taking the mundane phenomenon of tickling and extracting a genuinely unsettling philosophical conclusion from it -- that tickling implies your body can be "hijacked" by external stimuli in ways that bypass your sense of self -- and then deflating the whole argument with baby talk. It is a classic SMBC pattern of building up intellectual rigor only to undercut it with absurdity.