But, further complexity still, how does heat act in this primordial nebula? If heat is a ‘mode of motion,’ by what were the heat vibrations wrought? You must have _then_, for the phenomenon of heat, what is indispensable to the physical theory of it now:—the inconceivable but indispensable ether of modern physics. But how came the ether in, and beyond, the cosmic cloud? It is an ‘existence,’ manifestly; more, it is ‘matter;’ but matter that transcends the range of the action of gravity; it is without weight, and differentiated from all matter that we know in a manner that thought cannot follow. How did this ‘existence’ pass from the imperceptible to the perceptible? and by its own requirement should not a complete philosophy furnish its past history? Let us come more closely into contact with the actual formula of this philosophy. ‘Evolution,’ says Spencer, ‘is an integration of matter and concomitant dissipation of motion, during which the matter passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity, and during which the retained motion undergoes a parallel transformation.’[7] Here, then, are the factors of the potential universe. By definition they are the sole structural essentials. Beyond them nothing should be asked, required, or assumed. What are these irreducible factors of the formula? ‘Matter’ and ‘motion.’ But we have clearly seen that we are cognizant of matter as such only by its ‘modes of motion’. It is _these_ for which evolution has to account. ‘Matter’ denuded of the qualities by which we now are cognizant of it, can be no other than dimensions without qualities, spatial presence undiscoverable by sense. On the other hand, pure motion, motion by itself, is impossible to thought. Motion is only known as an affection of matter. ‘It becomes manifest,’ says Spencer, ‘that our experience of _force_ is that out of which our idea of matter is built’.[8] But, in the terms of the formula, force can be nothing but matter affected by motion. What, then, was this primary matter on which motion first acted, and before motion had by various ‘modes’ produced in it a single quality? No answer is possible; but, nevertheless, as the philosophy unfolds itself we find that ‘we need not refrain ... from dealing with matter as made up of extended and resistant atoms,’[9] and henceforth we are led to consider the cosmic nebula as not only homogeneous, but as being, in its homogeneity, atomic. [Footnote 7: _First Principles_, p. 396.] [Footnote 8: _First Principles_, p. 167.] [Footnote 9: _First Principles_, p. 176.]