"Clairvoyant, n.: A person, commonly a woman, who has the power of seeing that which is invisible to her patron - namely, that he is a blockhead"

This quote by Ambrose Bierce is a humorous take on the concept of clairvoyance. Bierce is suggesting that a clairvoyant is somebody who can see the fact about their client, which is that they are a blockhead. This indicates that the patron is not aware of their own absurdity, and the clairvoyant has the ability to see this. Bierce is satirizing the concept of clairvoyance, as it is typically viewed as a mysterious and supernatural power. He is recommending that the power of clairvoyance is not a lot about seeing the future, but rather seeing the fact about the present. In this case, the truth is that the client is a blockhead. Bierce's quote is an amusing method of mentioning the paradox of the circumstance.