Is it not rather true, that we know nothing
at all about it, but what it is just as easy to tell as to think? We know
it is that reflexible medium which enables us to see; and this is
definition enough for all but the natively blind, to whom no definition
perhaps can ever convey an adequate notion of its use in respect to sight.

13. If a person cannot tell what a thing is, it is commonly considered to
be a fair inference, that he does not know. Will any grammarian say, "I
know well enough what the thing is, but I cannot tell?" Yet, taken upon
this common principle, the authors of our English grammars, (if in framing
their definitions they have not been grossly wanting to themselves in the
exercise of their own art,) may be charged, I think, with great ignorance,
or great indistinctness of apprehension; and that, too, in relation to many
things among the very simplest elements of their science. For example: Is
it not a disgrace to a man of letters, to be unable to tell accurately what
a letter is? Yet to say, with Lowth, Murray, Churchill, and a hundred
others of inferior name, that, "_A letter_ is _the first principle_ or
_least part_ of a word," is to utter what is neither good English nor true
doctrine. The two articles _a_ and _the_ are here inconsistent with each
other. "_A_ letter" is _one_ letter, _any_ letter; but "_the first
principle_ of a word" is, surely, not one or any principle taken
_indefinitely_. Equivocal as the phrase is, it must mean either _some
particular principle_, or some particular _first_ principle, of a word;
and, taken either way, the assertion is false. For it is manifest, that in
_no sense_ can we affirm of _each_ of the letters of a word, that it is
"_the first principle_" of that word. Take, for instance, the word _man_.
Is _m_ the first principle of this word? You may answer, "Yes; for it is
the first _letter_." Is _a_ the first principle? "No; it is the _second_."
But _n_ too is a letter; and is _n_ the first principle? "No; it is the
_last_!" This grammatical error might have been avoided by saying,
"_Letters_ are the first principles, or least parts, of words." But still
the definition would not be true, nor would it answer the question, What is
a letter? The true answer to which is: "A letter is an alphabetic
_character_, which commonly represents some elementary sound of human
articulation, or speech."

14. This true definition sufficiently
distinguishes letters from the marks used