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The concept of space therefore makes possible the intuition of external phenomena; but these phenomena to be realised must appeal to one of our senses, and this connecting link between the outer world and our consciousness is the concept which we call time. Quoting again from Kant:
Time is the formal condition, a priori, of all phenomena whatsoever. But, as all representations, whether they have for their objects external things or not, belong by themselves, as determinations of the mind, to our inner state; . . . therefore, if I am able to say, a priori, that all external phenomena are in space, I can, according to the principle of the internal sense, make the general assertion that all phenomena, that is, all objects of the senses, are in time, and stand necessarily in relations of time.
It follows, then, that our simplest possible expression for phenomena will be in terms of space and time, and that beyond this the human mind cannot go.
Turning here from metaphysical to scientific language, we speak of space and time as the fundamental units from which we deduce the laws of the external world. The fact that space appeals to us only through time furnishes us with our concept or unit of motion, which is the ratio of space to time. The external phenomena so revealed to us we call the manifestations of mass or energy, thus providing ourselves with a second unit. It must be observed, however, that mass or energy is not a new concept, but bears precisely the same relation to motion as Kant's Ding-an
sich bears to space and time: it is the unknowable cause of motion,-or more properly speaking it is the ability residing in an object to change the motion of another object and is measured by the degree of change it can produce. And I say mass or energy, advisedly, for the two are merely different names or different views of the same thing; we cannot conceive of matter without energy or of energy without matter. Our choice between the two depends solely on the simplicity and convenience with which deductions may be made from one or the other. From a physical standpoint the concept energy is rather the simpler, but mathematically our deductions flow more readily from the concept mass.
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 pres
sure 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
The desire of peace, as the world has known it in past times, signified always a turning away from the flotsam and jetsam of time and an attempt to fix the mind on absolute rest and unity,—the desire of peace has been the aspiration of faith. And because the object of faith cannot be seen by the eyes of the body or expressed in terms of the understanding, a firm grasp of the will has been necessary to keep the desire of the heart from falling back into the visible, tangible things of change and motion. For this reason, when the will is relaxed, doubts spring up and men give themselves wholly to the transient intoxication of the senses. Yet blessed are they that believe and have not seen. It was the peculiar quest of the nineteenth century to discover fixed laws and an unshaken abiding place for the mind in the very kingdom of unrest; we have sought to chain the waves of the sea with the winds.
And how does all this affect one who stands apart, striving in his own small way to live in the serene contemplation of the universe? I cannot doubt that there are some in the world to-day who look back over the long past and watch the toiling of the human race toward peace as a traveller in the Alps may with a telescope follow the mountain-climbers in their slow ascent through the snows of Mont Blanc; or again they watch our labours and painstaking in the valley of the senses and wonder at our grotesque industry; or look upon the striving of men to build a city for the
soul amid the uncertainties of this life, as men look at the play of children who build castles and domes in the sands of the seashore and cry out when the advancing waves wash all their hopes away. I think there are some such men in the world to-day who are absorbed in the fellowship of the wise men of the East, and of the no less wise Plato, with whom they would retort upon the accusing advocates of the present: "Do you think that a spirit full of lofty thoughts, and privileged to contemplate all time and all existence, can possibly attach any great importance to this life?" They live in the world of action, but are not of it. They pass each other at rare intervals on the thoroughfares of life and know each other by a secret sign, and smile to each other and go on their way comforted and in better hope.