Einstein’s puzzling over the apparent identity of the two forms of mass is related to what we now call the principle of equivalence. This principle can be formulated in several ways. One way is to insist that the two types of mass are identical because of a fundamental symmetry in nature—that the laws of physics must take the same form whether you are in a gravitational field or in a region of space with no gravity but, say, in a spaceship undergoing an equivalent acceleration.
