300 DISCUSSIONS IN COSMOLOGY. 



If the gravitation theory fails to explain the origin 

 of the sun, it fails yet more decidedly to account for 

 the nebulse. In fact it does not attempt any explana- 

 tion of the origin of the latter; for it begins by assuming 

 their existence, and not only so, but that they are in 

 process of condensation. This must be the case, because 

 the theory in question assumes that the particles of a 

 nebulous mass have, in virtue of gravity, a mutual 

 tendency to approach one another; and it cannot tell 

 us how this tendency could exist without producing its 

 effect. The advocates of the theory are not at liberty 

 to call in the aid of heat in order to explain why the 

 particles are not mutally approaching; because it is 

 this mutual approach which, according to the theory, 

 produces the heat, and of course without such approach 

 ^no heat could be generated. A nebulous mass with a 

 'tendency to condensation could not have existed from 

 eternity as such ; but what the previous condition of a 

 nebula was, and how it came to assume its present 

 state, the gravitation theory cannot say. It begins 

 with a star or sun in process of formation, but does not 

 help us to understand how the process of formation 

 commenced. 



It is quite otherwise, however, with the other theory. 

 This latter does not, like the former, begin by assuming 

 the existence of a nebulous mass; on the contrary, it 

 goes back to the very commencement of physical 

 inquiry, to the very point where physical investigation 

 takes its rise, and beyond which we cannot penetrate. 

 The only assumption it makes is that of the existence 

 of matter and motion — if indeed this can be called an 

 assumption. How matter and motion began to be, 

 whether they were eternal or were created, are ques- 

 tions wholly beyond the domain of the physicist. The 

 theory takes for a fact the existence of stellar masses 



