That the present knowledge of the world of science compels a faith in an all-pervading substance, of marvelous properties, and of intimate relationship to all forms of energy, is shown by the following quotations from Lord Kelvin, who is generally regarded as the world's greatest physicist: "The luminferous ether, that is the only substance we are confident of in dynamics. One thing we are sure of, and that is the reality and substantiality of the luminiferous ether." "What can this luminiferous ether be? It is something that the planets move through with the greatest ease. It permeates our air; it is nearly in the same condition, so far as our means of judging are concerned, in our air and in the interplanetary space." "You may regard the existence of the luminiferous ether as a reality of science." "It is matter prodigiously less dense than air--of such density as not to produce the slightest resistance to any body going through it."[A] [Footnote A: Kelvin's Lectures, Vol. 1, pp. 317, 334, 336, 354.] The theory of the ether is one of the most helpful assumptions of modern science. By its aid the laws of energy have been revealed. There is at the present time no grander or more fundamental doctrine in science than that of the ether. The nature of the ether is, of course, far from being clearly understood, but every discovery in science demonstrates that the hypothetical ether stands for an important reality of nature. Together with the doctrines of the indestructibility of matter and energy, the doctrine of the ether welds and explains all the physical phenomena of the universe. Joseph Smith, in a revelation received on December 27, 1832, wrote: [Sidenote: Joseph Smith taught space is filled with a substance comparable to the ether of science.] "The light which now shineth, which giveth you light, is through him who enlighteneth your eyes, which is the same light that quickeneth your understandings; which light proceedeth forth from the presence of God to fill the immensity of space. The light which is in all things: which is the law by which all things are governed: even the power of God."[A] [Footnote A: Doctrine and Covenants, section 88:11-13.] This quotation gives undoubted evidence of the prophet's belief that space is filled with some substance which bears important relations to all natural phenomena. The word substance is used advisedly; for in various places in the writings of Joseph Smith, light, used as above in a general sense, means spirit,[A] and "all spirit is matter, but it is more fine and pure."[B] [Footnote A: Doctrine and Covenants, 84:45.] [Footnote B: Ibid, 131:7.]