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.