Likutay Moharan - Volume 2 - Torah-teaching 50

Likutay Moharan II, Torah 50: Thought Is in the Hand of a Person to Direct It as He Wishes

Machshavah [thought] is in the hand of a person to direct it to the place he wishes. And it is impossible for two machshavos [thoughts] to be together at all. And even if sometimes his machshavah flies and wanders to other and foreign things, it is in the hand of the person to direct it against its will to the straight path to think what is proper. And it is actually like a horse that turns from the path and veers to another path, that they seize it by its reins and return it against its will to the straight path. Similarly, one can seize the machshavah against its will to return it to the proper path.