How to control your brain at will By Dr.Roger vittoz and Christian H. godefroy

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Concentration is the faculty of being able to fix thoughts on a given point, to develop an idea without getting distracted, to be able to lose oneself in a book, in some kind of work, etc. the faculty is completely lacking in neurasthetic patients. 

We can define willpower as a separate force, a special energy existing in each individual, independent of any thought or idea, which manifests itself under certain conditions which we will specify in a moment.