When one of the company said to him, “convince me that logic is useful,” he said, … Would you like me to demonstrate it to you? Then I must make use of a demonstrative argument? And how will you know, then, whether I am deceiving you with a sophism? And when the man remained silent, he said, … You see how you yourself admit that logic is necessary, since without it you cannot even determine whether it is necessary or not.

~ Epictetus