Though the word "absolute" in the sense of "not implying any relations" has lost its original meaning, it is still freely used. It is proposed to define an absolute system of reference in the new sense as one which is privileged, because it gives rise to a maximum of simplicity for the expression of-the laws of nature. This definition is shown to be in agreement with the practical use of the word. Systems of reference in classical mechanics and systems o~[ units in physics are discussed from this point of view.
I. Introduction. Practical Definition o/ "Absolute Systems" and
Theorem. It is well known that N e w t o n attached a great meaning to the absolute in its true sense. He firmly believed that his idea of absolute motion had to be taken literally, that is in the sense of not implying any relations. In more modern times, from about 1870 on, his ideas in this respect were more and more submitted to criticism, and at present it is generally recognised that the absolute in the true sense does not deserve a place in mechanics or physics.
Nevertheless we are faced with the fact that the term "absolute" is freely used in physics and mechanics: one speaks, be it with some hesitation, of absolute motion, and the terms absolute units and absolute temperature are quite common. In all such cases there is a basic system of reference: for the motion it is a system of axes, for the units a system of basic units, and for the temperature it is a point of reference known as the absolute zero. Because of the high importance of these so-called absolute systems of reference, we propose to retain the word "absolute" for them. This need not give rise to any confusion, as long as one remains aware of the fact that such systems can be defined in a perfectly relative manner and that the --