In order to appreciate the fundamental ideas involved, suppose for a moment that gravitation could be annihilated, completely, and suppose I find myself upon this earth in empty space. You shall be seated at some point in space and shall watch my doings. If I am in the condition of mind of the people of the reign of King Henry VIII, I shall believe that the earth does not rotate. If I let go a stone, there being no gravity, I shall find that it flies away from me with an acceleration. You will know, however, that the stone really moves in a straight line with constant velocity, and that the apparent acceleration which I perceive is due to the earth's rotation. If I have argued that acceleration is due to force, I shall say that the earth repels the stone, and shall try to find the law governing the variation of this force with distance. I may go farther, and try to imagine some reason for the force, some pushing action transmitted from the earth to the stone through a surrounding medium; and, you will pity me for all this wasted labor, and particularly for my attempt to find a mechanism to account for the force, since you know that if I would only accept your measurements all would appear so simple. Let us probe this matter a little farther, however, from the stand-point of myself. I must believe in the reality of the force, since I have to be tied to my chair to prevent my departure from the earth. I might wonder how this field of force would affect the propagation of light, chemical action and so forth. For, even though I had discovered that, by using your measures, I could transform away the apparent effects of my field of force as far as concerned its power to hurl stones about, I could still regard this as a mathematical accident, and believe that the force was really there. Although I might suspect that the same transformation of view-point that would annul the field's effect as regards the stones would also annul its effect as regards light, etc., I should not be sure of this, as you would be; and my conscience would hardly allow me to do more than look upon the assumption of complete equivalence between the apparent field and a change in the system of measurement as a hypothesis. I should be strongly tempted to make the hypothesis, however.