Of all the sources of faulty and misleading reasoning, ambiguity is the most fruitful and the most inclusive.

It springs from the facts that words, except those which are almost technically specific, are constantly used in more than one sense, and that a great many of the words which we use in everyday life are essentially vague in meaning.  Such common words as “liberty,” “right,” “gentleman,” “better,” “classic,” “honor,” and innumerable others each need a treatise for any thorough definition; and then the definition, if complete, would be largely a tabulation of perfectly proper senses in which the words can be used, or a list of the ways in which different people have used them.  Besides this notorious vagueness of many common words, a good many words, as I have already shown (p. 54), have two or more distinct and definable meanings.

Strictly speaking, the ambiguity does not inhere in the word itself, but rather in its use in an assertion, since ambiguity can arise only when we are making an assertion.  It has been defined as “the neglect of distinctions in the meaning of terms, when these distinctions are important for the given occasion."[48] Suppose, for example, you are arguing against a certain improvement in a college dormitory, on the ground that it makes for luxury:  clearly “luxury” is a word that may mean one thing to you, and another to half of your audience.  By itself it is an indefinite word, except in its emotional implication; and its meaning varies with the people concerning whom it is used, since what would be luxury for a boy brought up on a farm would be bare comfort to the son of wealthy parents in the city.  Indeed the advances of plumbing in the last generation have completely changed the relative meanings of the words “comfort” and “luxury” so far as they concern bathrooms and bathtubs.  In the case of such a word, then, the weight of the definition above falls on the last clause, “when these distinctions are important for the given occasion”; here is a case where the occasion on which the word “luxury” is used determines nearly the whole of its meaning.  In practice, if you have a suspicion that a word may be taken in another sense than that you intend, the first thing to do is to define it—­to lay down as exactly as possible the cases which it is intended to cover on the present occasion, and the meaning it is to have in those cases.  For good examples of this enlightened caution, see the definitions on pages 54-65, especially that from Bagchot.

A similar difficulty arises with the words which, in the somewhat slipshod use of everyday life, have come to have as it were a sliding value.

We may raise no difficulty about understanding the assertions that Brown, and Jones, and Robinson are “honest,” but when we come to the case of Smith we discover a difficulty in placing him clearly on either side of the line.  That difficulty is nothing less than the difficulty of knowing the meaning given to the word in this particular assertion.  We might, for instance, agree to mean by Smith’s “honesty” that no shady transactions could be legally proved against him, or that he is “honest according to his lights,” or again that he is about as honest as the majority of his neighbors or the average of his trade or profession.[49]