ing, so far
as the mind of the learner is concerned, is a growth; and teaching, so
far as the teacher is concerned, is doing whatever is necessary to cause
that growth.

Let us proceed a step farther in this matter.

One of the ancients observes that a lamp loses none of its own light by
allowing another lamp to be lit from it. He uses the illustration to
enforce the duty of liberality in imparting our knowledge to others.
Knowledge, he says, unlike other treasures, is not diminished by giving.

The illustration fails to express the whole truth. This imparting of
knowledge to others, not only does not impoverish the donor, but it
actually increases his riches. _Docendo discimus._ By teaching we learn.
A man grows in knowledge by the very act of communicating it. The reason
for this is obvious. In order to communicate to the mind of another a
thought which is in our own mind, we must give to the thought definite
shape and form. We must handle it, and pack it up for safe conveyance.
Thus the mere act of giving a thought expression in words, fixes it more
deeply in our own minds. Not only so; we can, in fact, very rarely be
said to be in full possession of a thought ourselves, until by the
tongue or the pen we have communicated it to somebody else. The
expression of it, in some form, seems necessary to give it, even in our
own minds, a definite shape and a lasting impression. A man who devotes
himself to solitary reading and study, but never tries in any way to
communicate his acquisitions to the world, or to enforce his opinions
upon others, rarely becomes a learned man. A great many confused, dreamy
ideas, no doubt, float through the brain of such a man; but he has
little exact and reliable knowledge. The truth is, there is a sort of
indolent, listless absorption of intellectual food, that tends to
idiocy. I knew a person once, a gentleman of wealth and leisure, who
having no taste for social intercourse, and no material wants to be
supplied, which might have required the active exercise of his powers,
gave himself up entirely to solitary reading, as a sort of luxurious
self-indulgence. He shut himself up in his room, all day long, day after
day, devouring one book after another, until he became almost idiotic by
the process, and he finally died of softening of the brain. Had he been
compelled to use his mental acquisitions in earning his bread, or had
the love of Christ constrained him to use them in the instruction o