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 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
dedication 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
-- Strunk and White, "The Elements of Style"