he can know exactly how much and what he is to understand by our words.  The power which we possess, of making known all our complex or general ideas of things by means of definitions, is a faculty wisely contrived in the nature of language, for the increase and spread of science; and, in the hands of the skillful, it is of vast avail to these ends.  It is “the first and highest philosophy,” instructing mankind, to think clearly and speak accurately; as well as to know definitely, in the unity and permanence of a general nature, those things which never could be known or spoken of as the individuals of an infinite and fleeting multitude.

8.  And, without contradiction, the shortest and most successful way of teaching the young mind to distinguish things according to their proper differences, and to name or describe them aright, is, to tell in direct terms what they severally are.  Cicero intimates that all instruction appealing to reason ought to proceed in this manner:  “Omnis enim quse a ratione suscipitur de re aliqua institutio, debet a definitione proficisci, ut intelligatur quid sit id, de quo disputetur.”—­Off.  Lib. i, p. 4.  Literally thus:  “For all instruction which from reason is undertaken concerning any thing, ought to proceed from a definition, that it may be understood what the thing is, about which the speaker is arguing.”  Little advantage, however, will be derived from any definition, which is not, as Quintilian would have it, “Lucida et succincta rei descriptio,”—­“a clear and brief description of the thing.”

9.  Let it here be observed that scientific definitions are of things, and not merely of words; or if equally of words and things, they are rather of nouns than of the other parts of speech.  For a definition, in the proper sense of the term, consists not in a mere change or explanation of the verbal sign, but in a direct and true answer to the question, What is such or such a thing?  In respect to its extent, it must with equal exactness include every thing which comes under the name, and exclude every thing which does not come under the name:  then will it perfectly serve the purpose for which it is intended.  To furnish such definitions, (as I have suggested,) is work for those who are capable of great accuracy both of thought and expression.  Those who would qualify themselves for teaching any particular branch of knowledge, should make it their first concern to acquire clear and accurate ideas of all things that ought to be embraced in their instructions.  These ideas are to be gained, either by contemplation upon the things themselves as they are presented naturally, or by the study of those books in which they are rationally and clearly explained.  Nor will such study ever be irksome to him whose generous desire after knowledge, is thus deservedly gratified.