There is an agreement among philosophers and scientists that the concept of space is not derived from external experience, but is inherently intuitive. As stated by Kant: The representation of space cannot be borrowed through experience from relations of external phenomena, but, on the contrary, those external phenomena become possible only by means of the representation of space. Space is a necessary representation, _a priori_, forming the very foundation of external intuitions. It is impossible to imagine that there should be no space, though it is possible to imagine space without objects to fill it. 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.