$5 l ' r The ^Theory of the -Aether 



The history of this problem may be traced back continuously 

 to the earlier half of the seventeenth century. It first emerged 

 clearly in that reconstruction of ideas regarding the physical 

 universe which was effected by Eene Descartes. 



Descartes was born in 1596, the son of Joachim Descartes, 

 Counsellor to the Parliament of Brittany. As a young man he 

 followed the profession of arms, and served in the campaigns of 

 Maurice of Nassau, and the Emperor ; but his twenty-fourth 

 year brought a profound mental crisis, apparently not unlike 

 those which have been recorded of many religious leaders ; and 

 he resolved to devote himself thenceforward to the study of 

 philosophy. 



The age which preceded the birth of Descartes, and that in 

 which he lived, were marked by events which greatly altered 

 the prevalent conceptions of the world. The discovery of 

 America, the circumnavigation of the globe by Drake, the over- 

 throw of the Ptolemaic system of astronomy, and the invention 

 of the telescope, all helped to loosen the old foundations and to 

 make plain the need for a new structure. It was this that 

 Descartes set himself to erect. His aim was the most ambitious 

 that can be conceived ; it was nothing less than to create from 

 the beginning a complete system of human knowledge. 



Of such a system the basis must necessarily be metaphysical ; 

 and this part of Descartes' work is that by which he is most 

 widely known. But his efforts were also largely devoted to the 

 mechanical explanation of nature, which indeed he regarded as 

 one of the chief ends of Philosophy.* 



The general character of his writings may be illustrated by 

 a comparison with those of his most celebrated contemporary, f 

 Bacon clearly defined the end to be sought for, and laid down 

 the method by which it was to be attained; then, recognizing 

 that to discover all the laws of nature is a task beyond the 



* Of the works M'hich bear on our present subject, the Dioptrique and the 

 Me'teores were published at Leyden in 1638, and the Principia Philosophiae at 

 Amsterdam in 1644, six years before the death of its author. 



t The principal philosophical works of Bacon were written about eighteen years 

 before those of Descartes. 



