. . . . Berkeley's criticism of Newton's theory [in De Motu (1721) ] moves on two planes. With regard to the method of physics he returns to the principle of relativity. A body can be recognized as moving only in relation to another body that is relatively at rest. The idea of absolute motion is incompatible with the conditions of experience. . . .
Berkeley's second approach to the problem lies on the way of a psychological analysis of the illusions that lead to the assumption of absolute space. The idea of a space without a content is empty, it is a merum nihil. We are deceived, however, into the assumption because in speculating on the problem of space we subtract all bodies but forget to subtract our own. If we imagine space emptied of all content we still have an experience of space because we have the experience of our body and of the movements of its members. . . . Berkeley [in the Principles of Human Knowledge ] recognizes the experience of absolute motion [as when taking a walk], but he considers it impermissible to inject this experience into mathematical physics.
. . . . A psychological analysis of the Berkeleyan type . . . . cannot, however, persuade a physicist to consider his problem solved. . . . In brief: the criticism of the philosophers, as Berkeley did with Newton, is not constructive. As far as physics is concerned, the only result will be that the physicists will have to put them in their place. And this is what actually happened through Leonhard Euler in his Reflections sur léspace et le temps (1748). The philosophers were told that the certainty of the laws of mechanics must be the starting point of the inquiry. Any criticism that is in conflict with those principals must be rejected, however conclusive in itself it may be.
. . . . If we take Euler's demand seriously and generalize it, we arrive at the rule that every time an empirical scientist makes a mess of his fundamental concepts —which is a rather ordinary occurrence— the philosophers would be faced by the alternative of either clearing up the mess for him, or henceforth talking nonsense in epistemology and metaphysics. The demand has a touch of the burlesque. Nevertheless, it could be imposed with a measure of success. The graveness of the situation may be gathered from the fact that even Kant submitted to it. . . .