This book examines the method of mental variation, traditionally used predominantly in phenomenology and philosophy of science. The chapters include a discussion of Lockes 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 Husserls idea of free variation, and Reinachs extension of Husserls method. Finally, there is a discussion of variation in Cognitive Science as well as a discussion of Ernest Machs understanding of variation in thought experiments.