This cartoon appears in the book Computation Engineering:
Applied Automata Theory and Logic by Dr. Ganesh Gopalakrishnan.
I drew this comic at the suggestion of my computational theory instructor, who thought that there was potential humor in the conundrum:
Every barber shaves everyone who does not shave himself. No barber shaves someone who shaves himself. Prove that there exists no barber.The trick is -- if you follow this statement to its conclusion using formal logic, then you really can prove that barbers don't exist. Absurd, you say. The flaw lies not with our powers of deduction, but rather with the premise. This is a trivial example, pertinent only in the minds of theoreticians. But there is a lesson to be learned here... no matter how carefully we may analyze something, we can still go astray if our initial assumptions are not correct.