4^6 



ON THE HYPOTHESIS 



bly more so, instead of less so, than that of other persons ; since he 

 said to obtain it from a faculty which is not supposed to be injured, and 

 since the want of one sense is usually found to strengthen the remainder. 



With respect to the idea of extension, indeed, which, by some philoso- 

 phers, is thought to be most difficult of the whole, it appears to me that 

 it is capable of being obtained with at least as much perspicuity as that 

 of most other qualities of bodies, and more so than ideas of any of them ; 

 for we have in this instance the power of touch to correct that of sight, 

 or vice versa ; while in a multitude of other instances we are compelled 

 to trust to one sense alone. Extension, in its general signification, is a 

 complex idea, resulting from a combination of the more simple ideas of 

 length, breadth, and thickness ; and hence evidently imports a continuity 

 of the parts of whatever subject the idea ifs applied to ; whether it be a 

 solid substance, as a billiard ball, or the unsolid space which measures the 

 distance between one bilhard ball and another ; the idea of wea^wre being, 

 indeed, the most obvious idea we can form of it. In both which cases 

 we determine the relative proportions of the length, breadth, and thick- 

 ness by the eye, by the touch, or by both : and acquire, so far as I can see 

 to the contrary, notwithstanding all that has been said upon the subject, as 

 clear an idea as we do of substance. It is first obtained, I grant, from the 

 sight or touch of what is solid alone ; and it is afterwards made use of in 

 a more abstract form, as a measure of what is unsolid ; whence the mind 

 is able to apply it not only to the subject of pure space, but to a contem- 

 plation of circles, triangles, polygons, or any other geometrical figure, even 

 though such figures be not present to the senses, and exist alone in its 

 own conceptions. 



Extension, by the Cartesian school, was only applied to solid substance, 

 or body ; but then they supposed the universe to consist of nothing but 

 solid substance, or body, and that there is no such thing as vacuum, or 

 pure space. Among the Newtonians, who admit space, extension is ap- 

 plied as generally to this latter as to the former ; but in order to avoid 

 the confusion to which the application of this term to things so totally 

 opposite as matter and space has protluced in common -Jjscourse, Mr. Locke 

 advises to appropriate the term extension to body, and expansion to space ; 

 using both these terras, however, as perfect synonyms, and as equally im- 

 porting the simple idea of measure; which, as 1 have just observed, is the 

 most obvious and explanatory idea that can be offered upon this subject. 



Widely different, however, is the opinion of the metaphysic<al school of 

 North Britain ; and hence, in order to account for these abstruse ideas, to 

 which they affirm that neither our senses nor our reason can give rise, as 

 also in order to compel our behef that the external world exists in every 

 respect precisely as it appears to exist, and that external bodies possess 

 in themselves all the qualities, both primary and secondary, which they 

 APPEAR TO POSSESS, aod thus, with one wide sweep, to clear the ground 

 as well of the errors of Des Cartes, Newton, and Locke, as of those of 

 Berkeley and Hume ; Dr. Reid, who, at one time, had been a follower of 

 Berkeley, and, as he himself tells us, had embraced the whole of his sys- 

 tem,"* steps forth with his new theory, the more important doctrines of 

 which may be comprised under the four following heads :~ 



I. There exist in the mind of man various ideas or conceptions, both 



* See DngaUl Stewai t's Essay Note E. p, 543., and compare ^?ith ch. i. pp. C2, 63, 



