He wrote one of Dan Passner's favorite essays, 1941's "Self-Hatred Among Jews," while he collaborated with his best-known student on the essay "Level of Aspiration."  
This man's work is discussed in a 1969 book by Alfred Marrow, which refers to him as a "practical theorist."  He wrote one of Dan Passner's favorite essays, 1941's "Self-Hatred Among Jews," while he collaborated with his best-known student on the essay "Level of Aspiration."  The concepts of "unfreezing" and "cognitive aids" play a major role in his theory of the "T-group."  More famously, a spiral of steps enables people to find the most effective form of social engagement in his so-called "action research."  FTP, name this student of Carl Stumpf and teacher of Leon Festinger who wrote A Dynamic Theory of Personality and Frontiers in Group Dynamics, a German who is considered the founder of modern social psychology.
This man's work is discussed in a 1969 book by Alfred Marrow, which refers to him as a "practical theorist."  
Kurt Lewin