This book originates with the first course I took in graduate school. The course was in the Philosophy of Science, and it was taught by Arthur Fine at Northwestern University. It was not the first time I have read Kuhn’s The Structure of Scientific Revolutions, but this time reading it I became intrigued with Kuhn’s claim that the concept of mass is a primary example of a concept that has undergone a scien- tific revolution. According to Kuhn, the parameter m in Newtonian physics and in the Special Theory of Relativity might be referred to with the same word, mass.