G.K. Chesterton on Defining TermsCharles Dickens: A Critical Study (Dodd, Mead & Company: 1906) p. 1.
Much of our modern difficulty, in religion and other things, arises merely from this, that we confuse the word “indefinable” with the word “vague.” If some one speaks of a spiritual fact as “indefinable” we promptly picture something misty, a cloud with indeterminate edges. But this is an error even in common-place logic. The thing that cannot be defined is the first thing; the primary fact. It is our arms and legs, our pots and pans, that are indefinable. The indefinable is the indisputable. The man next door is indefinable, because he is too actual to be defined. And there are some to whom spiritual things have the same fierce and practical proximity; some to whom God is too actual to be defined.But there is a third class of primary terms. There are popular expressions which everyone uses and no one can explain; which the wise man will accept and reverence, as he reverences desire or darkness or any elemental thing. The prigs of the debating club will demand that he should define his terms. And, being a wise man, he will flatly refuse. This first inexplicable term is the most important term of all. The word that has no definition is the word that has no substitute. If a man fall back again and again on some such word as ‘vulgar’ or ‘manly’, do not suppose that the word means nothing because he cannot say what it means. If he could say what the word means he would say what it means instead of the word. … Precisely because the word is indefinable, the word is indispensable.