Shall we carry over our idea (b) to answer the next question: "To which material body shall we attach our framework?" To this question Newton gave one answer and Einstein another. We shall first consider Newton's position and then we may hope to see clearly where the new theory diverges from the classical or Newtonian mechanics. Newton's answer was that there is a particular material frame with reference to which the laws of mechanics have a remarkably simple form commonly known as "Newton's laws of motion" and so it is preferable to use this framework which is called an absolute frame. What is the essential peculiarity of an absolute frame? Newton was essentially an empiricist of Bacon's school and he observed the following facts. Let us suppose we have a framework of reference attached to the earth. Then a small particle of matter under the gravitational influence of surrounding bodies, including the earth, takes on a certain acceleration $A_1$. Now suppose the surrounding bodies removed (since we cannot remove the earth we shall have to view the experiment as an abstraction), and another set introduced; the particle, being again at its original position, will begin to move with an acceleration $A_2$. If both sets of surrounding bodies are present simultaneously the particle begins to move with an acceleration which is approximately but not quite the sum of $A_1$ and $A_2$. Newton postulated there there is a certain absolute reference frame in which the approximation would be an equality; and so the acceleration, relative to the material frame, furnishes a convenient measure of the effect of the surrounding bodies--which effect we call their gravitational force. Notice that if the effect of the surrounding bodies is small the acceleration is small and so we obtain as a limiting case, Newton's law of inertia which says that a body subject to no forces has no acceleration; a law which, as Poincaré justly observed, can never be subjected to experimental justification. The natural questions then arise: which is the absolute and privileged reference-frame and how must the simple laws be modified when we use a frame more convenient for us--one attached to the earth let us say? The absolute frame is one attached to the fixed stars; and to the absolute or real force defined as above, we must add certain terms, usually called centrifugal forces. These are referred to as fictitious forces because, as it is explained, they are due to the motion of the reference-frame with respect to the absolute frame and in no way depend on the distribution of the surrounding bodies. Gravitational force and centrifugal forces have in common the remarkable property that they depend in no way on the material of the attracted body nor on its chemical state; they act on all matter and are in this way different from other forces met with in nature, such as magnetic or electric forces. Further Newton found that he could predict the facts of observation accurately on the