THE CONCEPT of mass is one of the most fundamental notions in physics, comparable in importance only to the concepts of space and time. Isaac Newton, who was the first to make systematic use of the concept of mass, was already aware of its importance in physics. It was probably not a matter of fortuity that the very first statement in his Principia, the most influential work in classical physics, presents his definition of mass or of “quantitas materiae,” as he still used to call it.1 However, his definition of mass as the measure of the quantity of matter, “arising from its density and bulk conjointly,” was for several reasons soon regarded as inadequate. Since then, the quest for an adequate definition of mass, combined with the search for a more profound understanding of its meaning, its nature, and its role in the physical sciences, has never ceased to engage the attention of physicists and philosophers alike.
That still today “mass is a mess,” as a contemporary physicist punningly phrased it,2 should not come as a surprise. For “in the world of human thought generally, and in physical science particularly, the most important and most fruitful concepts are those to which it is impossible to attach a well-established meaning.”3
Yet, the remarkable progress in experimental and theoretical physics made during the past few decades has considerably deepened our knowledge concerning the nature of mass. In particular, recent advances in the general theory of relativity and in the theory of elementary particles have opened new vistas that promise to lead us to a more profound understanding of the nature of mass. It is the intention of the present study to review these developments in a rigorous and yet concise fashion.____________________