The idea that our minds are capable of determining more of the outcome of our lives than if we just follow our instincts by merely reacting to stimuli.

Some manifestations of mental discipline:
One who has mental discipline is able to control their anger in any situation.
One who has mental discipline is able to control their emotions.
One who has mental disipline is rarely bored.
One who has mental discipline has mastered their own psyche, and is aware of what environmental factors could damage it (like, alcohol, for some), and controls the amount of exposure they get to such things.

People who are like this tend to appear calm all the time. They tend to face conflict with an attitude of "Ok, what's really the problem here and how can I fix it, deal with it, or work around it if it is impossible for me to change".

An example:
Viktor Frankl survived a Nazi concentration camp, after which he wrote a book entitled Man's Search for Meaning which detailed his physical and mental experience as a prisoner. The crux of his book is that no matter what they do to you, if you are still in control of your mind, you are surviving, you are still you.

He developed a field of psychotherapy called Logotherapy and Existential Analysis. The basis for his work is that "Persons are capable of deciding, therefore they are responsible for their decisions. A human being is not a mere puppet of biology, heredity and environmental forces, but is always free to take a stand toward inner conditions and outer circumstances".

The quote came from