10.  But it must be understood, that although scientific definitions are said to be of things, they are not copied immediately from the real essence of the things, but are formed from the conceptions of the author’s mind concerning that essence.  Hence, as Duncan justly remarks, “A mistaken idea never fails to occasion a mistake also in the definition.”  Hence, too, the common distinction of the logicians, between definitions of the name and definitions of the thing, seems to have little or no foundation.  The former term they applied to those definitions which describe the objects of pure intellection, such as triangles, and other geometrical figures; the latter, to those which define objects actually existing in external nature.  The mathematical definitions, so noted for their certainty and completeness, have been supposed to have some peculiar preeminence, as belonging to the former class.  But, in fact the idea of a triangle exists as substantively in the mind, as that of a tree, if not indeed more so; and if I define these two objects, my description will, in either case, be equally a definition both of the name and of the thing; but in neither, is it copied from any thing else than that notion which I have conceived, of the common properties of all triangles or of all trees.

11.  Infinitives, and some other terms not called nouns, may be taken abstractly or substantively, so as to admit of what may be considered a regular definition; thus the question, “What is it to read?” is nearly the same as, “What is reading?” “What is it to be wise?” is little different from, “What is wisdom?” and a true answer might be, in either case, a true definition.  Nor are those mere translations or explanations of words, with which our dictionaries and vocabularies abound, to be dispensed with in teaching:  they prepare the student to read various authors with facility, and furnish him with a better choice of terms, when he attempts to write.  And in making such choice, let him remember, that as affectation of hard words makes composition ridiculous, so the affectation of easy and common ones may make it unmanly.  But not to digress.  With respect to grammar, we must sometimes content ourselves with such explications of its customary terms, as cannot claim to be perfect definitions; for the most common and familiar things are not always those which it is the most easy to define.  When Dr. Johnson was asked, “What is poetry?” he replied, “Why, sir, it is easier to tell what it is not.  We all know what light is:  but it is not easy to tell what it is.”—­Boswell’s Life of Johnson, Vol. iii, p. 402.  This was thought by the biographer to have been well and ingeniously said.