If then our explanations of phenomena must ultimately involve the two units of motion and of energy or mass, and if it can be demonstrated that on this basis we may account for any group of phenomena in an infinite number of ways, what shall we say but that the attempt to attain any resting-place for the mind in the laws of nature is, and must always be, futile? Further than this, any given law is itself only an approximate explanation of phenomena, and must be continually modified as we add to our experimental knowledge. In all cases a law must be considered valid only within the limits of the sensitiveness of the instruments by which we get our measurements. With more delicate instruments variations will be observed that must be expressed by additional terms in the formula. Thus we maintain that the law of gravitation is true only within the range of our observation; it does not apply to masses of molecular dimensions. Another formula, the well-known law of the pressure of gases, can be shown by experiment to be merely an approximation, because the variations in it are not of a dimension negligible in comparison with the sensibility of our instruments. As the pressure increases the error in the formular equation becomes constantly greater. To remedy this a second approximation, which is still inadequate, has been added to the equation by Van der Waals; yet greater accuracy will require the addition of other terms; and a complete demonstration would demand an infinite series of approximations. The meaning of all this is quite plain: there is no reach of the human intellect which can bridge the gap between motion and rest. Our senses are adapted to a world of universal flux which is, so far as we can determine, subject to no absolute law but the law of probabilities. He who attempts to circumscribe the ebb and flow of circumstance within the bounds of our spiritual needs, he who attempts to find peace in any formula of science or in any promise of historic progress, is like one who labours on the old and vain problem of squaring the circle: Qual è'l geomètra, che tutto s'affige Per misurar lo cerchio, e non ritrova, Pensando, quel principio ond' egli indige.