Explain this epigram: "Sir, I admit your general rule, That every poet is a fool. But you yourself may serve to show it, That every fool is not a poet."

This epigram is a witty way of pointing out that while it may be true that not all poets are wise or intelligent, it is also true that not all foolish people are capable of being poets. In other words, just because someone may lack intelligence or wisdom does not automatically make them a poet. The speaker is using irony to challenge the assumption that all poets are fools, and suggesting that being a fool does not necessarily equate to being a poet.