tricks are played it is more and more difficult for the mind to tell the
truth. Such deceptions come through drunkenness and narcotism. In greater
or less degree all nerve-affecting drugs produce it; alcohol, nicotine,
caffeine, opium, cocaine, and the rest, strong or weak. Habitual use of any
of these is a physical vice. A physical vice becomes a moral vice, and all
vice leaves its record on the nervous system. To cultivate vice is to
render the actual machinery of our mind incapable of normal action.

It is the brain's business to perceive, to think, to will, to act. All
these functions taken together we call the mind. The brain is hidden in
darkness, sheltered within a box of bone. All that it knows comes to it
from the nerves of sense. All that it can do in this world is to act on the
muscles it controls through its nerves of motion. The final purpose of
knowledge is action. Our senses tell us what lies about us, that we may
move and act, and do this wisely and safely. The sense-organs are the
brain's only teachers so far as we know, the muscles are its only servants.
But there are many orders which may be issued to these servants. Out of the
many sensations, memories, imaginations, how shall the brain choose?

The power of attention fixes the mind on those sensations or impressions of
most worth, pushing the others into the background. Past impressions,
memory-pictures, linger in the brain, and these, bidden or unbidden, crowd
with the others. To know the relation of all these, to distinguish present
impressions from memories, realities from dreams,--this is mental sanity.
The sane brain performs its appointed task. The mind is clear, the will is
strong, the attention persistent, and all is well in the world. But the
machinery of the brain may fail. The mind grows confused. It mistakes
memories for realities. It loses the power of attention. A fixed idea may
take possession of it, or it may be filled by a thousand vagrant
impressions, wandering memories, in as many seconds. In this case the