There are two quite different kinds of difficulty in putting the right interpretation on a statement, and a dictionary can only remove one of these, and by far the less important one.  When you meet with a statement containing an unfamiliar word—­say, the word “parallax,” or “phanerogamous,” or “brigantine”—­and when you understand all the rest of the statement except that word, then as a general rule the dictionary will help to make the meaning clear.  But when the difficulty is caused, not by a word being unfamiliar, but by its being used in a certain context, then the best dictionary in the world is, for your purpose, of no use at all.  The nature of every dictionary is necessarily such that it entirely leaves out of account all doubts about meaning which are of this second kind.  The most that a dictionary can do is to tell us the meaning of a word in those cases where the context in which it is used is not such as to make the meaning doubtful.[8]

In practice the words which most often need definition are those which are, as it were, shorthand symbols for perhaps a very extensive meaning.  Unless the limits of this extended meaning are clearly marked out you cannot tell whether the minds of your readers are, as the lawyers say, running on all fours with your own or not.  This extended meaning may be of various sorts:  for example, it may be a large general principle, as in the case of “evolution” or “culture”; or it may be a general system or practice, as in the case of “commission government,” “honor system,” or “high standards for graduation”; or it may be a general class of things, persons, or events, as in the case of “secondary school,” “professional coach,” or “murder.”  When you use any such term in an argument, it is essential that your readers shall have the same set of details, ramifications, or instances in mind as you have yourself.  For this purpose you must define the term; or, in other words, you must lay out or display the ramifications and limitations of the principle, the details of the system or practice, or the exact kinds of things, persons, or events, which you have in mind when you use the term.  A few examples will make this practical meaning of defining clear.

Sometimes the definition proceeds by careful and specific limitation of the general signification of a word, as in the following example from Bagchot: 

I should say that except where it is explained to the contrary, I use the word “toleration” to mean toleration by law.  Toleration by society of matters not subject to legal penalty is a kindred subject, on which if I have room I will add a few words; but in the main I propose to deal with the simpler subject, toleration by law.  And by toleration, too, I mean, when it is not otherwise said, toleration in the public expression of opinions; toleration of acts and practices is another allied subject, on which I can, in a paper like this, but barely hope to indicate what seems to me to be