four
Explanation
This comic tackles the philosophical problem of epistemology -- specifically, how can you ever truly know the contents of another person's mind? This is sometimes called the "problem of other minds" in philosophy.
An older philosopher poses the classic question: "The central question of epistemology is: can you ever truly know the contents of another mind?" An engineer responds with a hilariously practical solution: "Yeah, it's easy. Have them think about the number 4 while shouting 'FOOOOUUUURRRRR! FOOOOUUUUURRRRR!'" -- and then you can be "pretty confident" they're thinking about four.
The caption at the bottom reads: "Engineers are no longer invited to the philosophy department." The joke plays on the fundamental clash between philosophical inquiry (which values deep, nuanced exploration of seemingly unanswerable questions) and engineering pragmatism (which values simple, workable solutions even if they completely miss the philosophical point). The engineer's "solution" technically works in a trivial sense but entirely sidesteps the actual depth of the epistemological question.