it is a conversation or a book, or a manual task, or a problem being
silently worked out intellectually, it should have undivided attention
until the mind is ready for something else.

Perhaps few of us give to any subject the close attention which alone
can prove its own effectiveness and demonstrate the fact that there
goes with such steadily sustained attention a subtle power of
extended, or accentuated, consciousness. When ten minutes is given to
a certain subject and other thoughts are constantly intruding, so that
when the ten minutes have passed only five minutes have actually been
devoted to the subject, the result is by no means a half of what would
have been accomplished had the whole of the ten minutes been given to
uninterrupted attention. The time thus spent in wavering attention is
practically without effect. The connection between mind and subject
has not been complete. Mind and subject were, so to say, out of focus.
Attention must be sustained to the point where it becomes
concentration. The mind must be used as a sun-glass can be used. Hold
the glass between sun and paper, out of focus, for an hour and nothing
will happen. A yellow circle of light falls on the paper and that is
all. But bring it into perfect focus, concentrating the rays to the
finest possible point, and the paper turns brown and finally bursts
into the fire that will consume it. They are the same rays that were
previously ineffective. Concentration produced results.

The mind must be brought under such complete control of the will that
it can be manipulated like a search-light, turned in this direction or
that, or flung full upon some obscure subject and held steadily there
till it illuminates every detail of it, as the search-light sends a
dazzling ray through space and shows every rock and tree on a hillside
far away through the darkness of the night.