Now the question raised by Einstein is whether the force of gravity, which we experience as a very real thing, may be put upon a footing which is in some way analogous to that of the obviously fictitious centrifugal force cited above: whether gravitation may be regarded as a figment of our imagination engendered by the way in which we measure things. He found that it could be so regarded. He went still farther, and in his Principle of Equivalence, he postulated that the apparent effects of gravitation in all phenomena could be attributed to the same change in the system of our measurements that would account for the ordinary phenomena of gravitation. On the basis of this hypothesis he was able to deduce for subsequent experimental verification, the effects of gravitation on light. He did not limit himself to such simple changes in our measurements as were sufficient to serve the purpose of the problem of centrifugal force cited above; but, emboldened by the assumptions, in the older theory of relativity, of change in standards of length and time on account of motion, he went even farther than this, and considered the possibility of change of our measures due to mere proximity to matter. His problem amounted to an attempt to find some way in which it was possible to conceive our scales and clocks as altered, relatively to some more fundamental set, so as to allow of the planetary motions being uniform and rectilinear with respect to these fundamental measures, although they appear as they do to us. If we allow our imaginations perfect freedom as to how the scales may be altered, we shall not balk at assuming alterations varying in any way we please, Einstein does, however, introduce restrictions for reasons which we will now discern.