ccompanied by corresponding qualities and quantities of
_mental_ activity. That is, when a person's muscle action, nerve
response, and brain action are sluggish, his _mind_ also develops a
characteristic of slow action. And vice versa.

We say of a certain acquaintance that he has an alert mind. But his
"ego," or mental self, could not act quickly and alertly if his _brain_,
the physical instrument of his _mind_, did not receive and transmit
impressions swiftly to his mentality. The _brain_ does not _think_. It
is as purely physical as any other part of the body. It just _handles_,
or transmits in and out, to and from the _mind_, the various impressions
sent _in_ by different sense muscles, and the mental reflexes or
impulses sent _out_ by the innumerable mind centers. Your mind works
_through_ your brain. Of course, therefore, the quality and quantity of
mental work _you_ are capable of doing are limited by the degree of
handling-or-transmitting _efficiency_ characteristic of _your_
particular brain structure.

[Sidenote: Value of Practical Psychology]

Any interference with the _brain_ quality or quantity of an individual
naturally interferes with his normal _mental_ functioning. If a
particular part of a man's brain is injured, the associated mind center
is harmed likewise and his mental _quality_ is affected in proportion.
Should a certain portion of his brain be cut out, the total _quantity_
of his mental powers would be correspondingly reduced. We all know these
things about the brain and the mind. But only a few scientists are
familiar with many _details_ of the _inter-relation of mind and brain
and muscles_, which should be known to all people who want to make the
most of themselves. The salesman of himself needs to understand his
"goods" thoroughly; so as we study the selling process that completes
the secret of certain success, we dig into _practical psychology_ a
little way now in order to stimulate in you a desire for further
exploration of that gold mine of opportunit