332 A SHORT HISTORY OF SCIENCE 



popular treatises on astronomy ever published, in which the great 

 mathematician never uses either an algebraical formula or a 

 geometrical diagram", Laplace presents the arguments for his 

 nebular hypothesis along the following general lines: 



In spite of the separation of the planets they bear certain re- 

 markable relations to each other ; 



All the planets travel about the sun in the same direction and 

 almost in the same plane ; 



The satellites also travel about their planets in this same 

 direction and almost in the same plane ; 



Finally, sun, planets and satellites revolve in the same sense 

 about their own axes and this rotation is approximately in the 

 orbital plane. 



These agreements cannot be accidental. Laplace seeks the 

 cause in the existence of an original vast nebulous mass forming 

 a sort of atmosphere about the sun and extending beyond the 

 outermost planet. Initial or acquired rotation of the nebula at- 

 tended by gradual cooling and contraction has caused the cen- 

 trifugal separation of masses analogous to Saturn's rings, out of 

 which planets have gradually condensed, throwing off their own 

 satellites in the process. This hypothesis had already been 

 proposed in substance by Kant in 1755. Its later history will 

 be touched on in a following chapter. 



Laplace was also deeply interested in the theory of probability, 

 as may be illustrated by the following passages : 



The most important questions of life are, for the most part, really 

 only problems of probability. Strictly speaking one may even say 

 that nearly all our knowledge is problematical; and in the small 

 number of things which we are able to know with certainty, even in 

 the mathematical sciences themselves, induction and analogy, the 

 principal means for discovering truth, are based on probabilities, so 

 that the entire system of human knowledge is connected with this 

 theory. 



It is remarkable that a science (probabilities) which began with 

 the consideration of games of chance, should have become the most 

 important object of human knowledge. 



