Your first impulse when you find a term that needs defining may be to go to a dictionary.  A little thought will show you that in most cases you will get little comfort if you do.  The aim of a dictionary is to give all the meanings which a word has had in reasonable use; what you need in an argument is to know which one of these meanings it has in the present case.  If you were writing an argument on the effects or the righteousness of the change wrought in the English constitution by the recent curtailment of the veto power of the House of Lords, and wished to use the word “revolution,” and to use it where it was important that your readers should understand precisely what you intended it to convey, you would not burden them with such a definition as the following, from an unabridged dictionary:  “Revolution:  a fundamental change in political organization, or in a government or constitution; the overthrow or renunciation of one government and the substitution of another, by the governed.”  Such a definition would merely fill up your space, and leave you no further ahead.  A dictionary is studiously general, for it must cover all possible legitimate meanings of the word; in an argument you must be studiously specific, to carry your readers with you in the case under discussion.

Moreover, words are constantly being pressed into new uses, as in the case of “commission” (see p. 54); and others have entirely legitimate local meanings.  Only a dictionary which was on the scale of the New English Dictionary and which was reedited every five years could pretend to keep up with these new uses.  In an unabridged dictionary dated 1907, for example, the full definition of “amateur” is as follows:  “A person attached to a particular pursuit, study or science, as to music or painting; especially one who cultivates any study or art, from taste or attachment, without pursuing it professionally.”  Of what use would such a definition be to you if you were arguing in favor of strengthening or relaxing the amateur rules in college athletics, in which you had to follow through the intricacies of summer baseball and of reimbursements for training table and traveling expenses?  Such a definition hardly comes in sight of the use of the word which is most in the mouths of college students in America.  Words mean whatever careful and accepted writers have used them to mean; and the business of a dictionary is so far as possible to record these meanings.  But language, being a living and constantly developing growth, is constantly altering them and adding to them.

What a dictionary can do for you, therefore, is merely to tell you whether in the past the word has been used with the signification which you wish to give to it; but there are very few cases in which this will be much help to you, for in an argument your only interest in the meaning of a term is in the meaning of that term for the case under discussion.