Relativism—Also known as the homo mensura (man is the measure) theory. Relativists reject any truth that is absolute. They argue that because each person sees things differently, truth exists individually for each person. It is therefore false to say one person is right and another is wrong. Relativism in Western philosophy is traced back to Protagoras, a contemporary of Socrates. See Absolute, Humanism. Relativity theory Albert Einstein compared his theory of relativity to a building with two stories. The ground floor is the special theory of relativity. It applies to all physical phenomena except gravitation. The general theory of relativity is the upper floor; it explains the law of gravitation. Einstein's theory combines two principles. One is that motion is relative. For example, when a table-tennis ball rolls across the surface of the playing table, its motion is relative to the table. In the classical physics of Newton, the rolling ball is considered to be the object in motion, and the table is considered to be at rest. However, if at first the ball was at rest on the tabletop, and we were to move the table, then both the table and the ball would move in relation to one another. Relativity theory argues that since all matter in the universe is in motion, the ball rolling upon the surface of a moving table is the actual model for motion in the universe. If space and time are taken to be the table, they contribute motion to the movement of all things. The old model of a ball rolling upon a tabletop at rest is therefore an illusion. The second principle of relativity is that the speed of light is always the same, even when light is emitted from a source that moves at a great speed towards, or away from, the observer. One of the significant differences between relativity theory and classical theory is seen in the calculation of the mass of a physical object. Mass is defined as the amount of matter in a physical object which is measured as that object's resistance to acceleration. Mass is different from, but proportional to, weight. Classical theory attributes a steady mass to any given physical object. Relativity theory predicts that the mass of a thing will vary according to its motion. Relativity is considered an advancement over, but not a replacement of, classical physics, which is still useful. From a logical point of view, the two theories are incompatible. The third important physical theory, quantum mechanics, is likewise incompatible with classical theory, and also differs significantly from relativity theory. See Quantum mechanical theory.