OF COMMON SENSE. 379 



length, breadth, and thickness by the eye, by the touch, or by both : and ac- 

 quire, so far as I can see to the contrary, notwithstanding 1 all that has been 

 said upon the subject, as clear an idea as we do of substance. It is first ob- 

 tained, I grant, from the sight or touch of what is solid alone ; and it is after- 

 ward 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 contemplation of circles, triangles, polygons, or any other geometrical 

 figuro, 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 applied 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 produced in common discourse, Mr. Locke advises to appropriate the 

 term extension to body, and expansion to space ; using both these terms, 

 however, as perfect synonyms, and as equally importing the simple idea of 

 measure; which, as I 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 metaphysical 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 belief that the external world exists in every respect 

 precisely AS IT APPEARS TO EXIST, and that external bodies possess in them- 

 selves all the qualities, both primary and secondary, which THEY APPEAR TO 

 POSSESS, and 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 him- 

 self tells us, "had embraced the whole of his system,"* 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 phy- 

 sical and metaphysical, which we have never derived either from sensation 

 or reflection. 



II. There must therefore exist, somewhere or other in the animal frame, a 

 third percipient principle, from which alone such ideas can have been den/ed 



III. From this additional principle there is no appeal : it is higher in its 

 knowledge, and surer in its decision, than either the senses or the reason ; it 

 compels our assent in a variety of cases, in which we should otherwise be 

 left in the most distressing doubt; and gives us an assurance, not only that 

 there is an external world around us, but that the primary and secondary 

 qualities of bodies exist equally and uniformly in the bodies themselves, or, 

 in other words, that every thing actually is as it appears to be. 



IV. This mandatory or superior principle is COMMON SENSE or INSTINCT. 

 And in order to ensure himself success in the establishment of the doctrines 



contained in this outline, Dr. Reid, with a warmer devotion than falls to the 

 lot of metaphysicians in general, and in some degree breathing of poetic 

 inspiration, opens his Inquiry with the following animated prayer : " Admired 

 philosophy! daughter of light! parent of wisdom and knowledge! if thou 

 art she ! surely thou hast not yet arisen upon the human mind, nor blessed us 

 with more of thy rays than are sufficient to shed a darkness visible upon the 

 human faculties, and to disturb that repose and security which happier mortals 

 enjoy, who never approached thine altar, nor felt thine influence ! But if, 

 indeed, thou hast not power to dispel those clouds and phantoms which thou 

 hast discovered or created, withdraw this penurious and malignant ray : I 

 despise philosophy, and renounce its guidance : let my soul dwell with com- 



Se Dugald Stewart's Essayi, note E, p. 543, and compare with ch. i. p. 62, 63 



