Someone once said to Epictetus, "Convince me that logic is necessary." Epictetus replied, "Do you want me to prove this to you?" The man said yes. "Then I'll have to use logical reasoning," Epictetus said. The man agreed. "But how will you know if I'm tricking you with my argument?" The man had no answer. "You see," said Epictetus, "you've just admitted that logic is necessary. Without it, you can't even tell whether logic itself is necessary or not."
When one of those who were present said, Persuade me that logic is necessary, he replied, Do you wish me to prove this to you? The answer was, Yes. Then I must use a demonstrative form of speech. This was granted. How then will you know if I am cheating you by my argument? The man was silent. Do you see, said Epictetus, that you yourself are admitting that logic is necessary, if without it you cannot know so much as this, whether logic is necessary or not necessary?