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.

12. But whenever we encounter difficulties of this sort, it may be worth
while to seek for their _cause_. If we find it, the understanding is no
longer puzzled. Dr. Johnson seemed to his biographer, to show, by this
ready answer, the acuteness of his wit and discernment. But did not the wit
consist in adroitly excusing himself, by an illusory comparison? What
analogy is there between the things which he compares? Of the difficulty of
defining _poetry_, and the difficulty of defining _light_, the reasons are
as different as are the two things themselves, _poetry_ and _light_. The
former is something so various and complex that it is hard to distinguish
its essence from its accidents; the latter presents an idea so perfectly
simple and unique that all men conceive of it exactly in the same way,
while none can show wherein it essentially consists. But is it true, that,
"We all know _what light is_?"