In a word, the masses of bodies are conventional data defined by the fact that they are proportional to the accelerations caused by one and the same force. To put it differently, the mass of a body is the quotient of the force which acts upon it by the acceleration given to it. Poincaré used to say picturesquely: “Masses are coefficients which it is convenient to use in calculations.” If there is one property of bodies which comes within the range of our senses, a property of which every man has some sort of instinct or intuition, it is _mass_. Yet careful analysis shows us that we are unable to define it otherwise than by disguised conventions. Poincaré’s definition seems paradoxical in its admission of powerlessness. But it is correct. Mass is only a “coefficient,” a conventional outcome of our weakness! Nevertheless, something remained upon which we thought we could base, if not our craving for certainty—genuine men of science gave up the idea of certainty long ago—at least our desire for accuracy of deduction in our classification of phenomena. We believed in the constancy of mass, of this convenient and so clearly defined _coefficient_. Here again, unfortunately, we have to recant—or, perhaps, we should say fortunately, as there is no pleasure like that of novelty. The older mechanics taught us that mass is constant in one and the same body, and is therefore independent of the velocity which the body acquires. From which it followed, as we have already explained, that, if a force continues to act, the velocity acquired at the end of a second will be doubled at the end of two seconds, tripled at the end of three seconds, and so on indefinitely. But we have just seen that the velocity increases less during the second second than during the first, and so on, continuously diminishing until, when the velocity of light is attained, that of the moving body can increase no further, whatever force may act upon it. What does that mean? If the velocity of a body increases less during the second second, it must be because it offers an increasing resistance to the accelerating force. Everything happens as if its inertia, its mass, had changed! Which amounts to saying that _the mass of bodies is not constant: it depends upon their velocity, and increases with an increase of velocity_. In the case of feeble velocities this influence is imperceptible. It was because the founders of classical mechanics, an experimental science, had experience only of relatively feeble velocities that they found that mass was _perceptibly_ constant, and believed they might conclude that it was _absolutely_ constant. In the case of greater velocities that is not so.