William Strunk, Jr.

If you have received a letter inviting you to speak at the dedication of a new cat hospital, and you hate cats, your reply, declining the invitation, does not necessarily have to cover the full range of your emotions. You must make it clear that you will not attend, but you do not have to let fly at the cats. The writer of the letter asked a civil question; attack cats, then, only if you can do so with good humor, good taste, and in such a way that your answer will be courteous as well as responsive. Since you are out of sympathy with cats, you may quite properly give this as a reason for not appearing at the dedicatory ceremonies of a cat hospital. But bear in mind that your opinion of cats was not sought, only your services as a speaker. Try to keep things straight.
— William Strunk, Jr., The Elements of Style
David Isaacson