No need for any commentary on this one; below is the full passage:
When someone who was attending his school said to him, ‘Convince me of the usefulness of logic,’ he replied: Would you like me to demonstrate it to you?—‘Yes.’ —Then I must employ a demonstrative argument? And when the questioner agreed, he asked: How will you know, then, whether I’m trying to mislead you with a sophism? The man offered no reply. So do you see, continued Epictetus, how you yourself are conceding that logic is necessary, since without it you can’t even tell whether it is necessary or not?