This book examines the method of mental variation, traditionally used predominantly in phenomenology and philosophy of science. The chapters include a discussion of Locke's understanding of how the mind works, and the possible role of variation in the empiricist model of acquisition of knowledge as well a discussion of Husserl's idea of free variation, and Reinach's extension of Husserl's method. Finally, there is a discussion of variation in Cognitive Science as well as a discussion of Ernest Mach's understanding of variation in thought experiments.