1886.] NATURAL SCIENCES OF PHILADELPHIA. 305 



must do so either in the mind or in external nature. If it be 

 internal, it must be of the nature of an idea an innate sense of 

 absolute direction, a mental power by which we can at once 

 distinguish the truth or falsity of any apparent direction. That 

 we have any such idea of the absolute, is in itself highly improb- 

 able. It may, in fact, be readily disproved. We are aware that 

 the revolution of the earth on its axis reverses the absolute direc- 

 tion of all the objects upon its surface every twelve hours. And 

 yet this reversal is not perceptible to us. The direction of all 

 objects, as related to that of our bodies, remains unchanged, and 

 this relation is all that we perceive. Indeed, we must have 

 remained forever ignorant of the revolution of the earth but from 

 the fact that the spheres of space do not revolve with it. These 

 retain their positions in space, while those of all objects upon the 

 earth change. They, therefore, serve us as standards of compari- 

 son by whose aid we intellectually discover the earth's motion. 

 We never become sensibly aware of it, for the only motion 

 apparent to us is that of the spheres of space. We impute 

 motion to objects at rest, and rest to objects in motion. This 

 error could not arise, had we any innate idea of absolute direc- 

 tion, or an internal standard of comparison. Therefore the mind 

 in itself is incapable of perceiving that there has been a reversal 

 of the retinal image. It has no faculty of deciding on questions 

 of direction, and what is relatively correct is absolutely correct 

 to its perceptive powers. 



If we have no internal or retinal standard of direction, have we 

 any external one ? No object upon the earth's surface will serve 

 this purpose. The images of all objects alike are reversed upon 

 the retina. The same is the case in regard to the spheres of space. 

 They do not retain their true positions in this case, as in the 

 former case considered, but are reversed in direction, and the 

 whole universe is turned upside down by the crystalline lens of 

 the eye, and is perceived by the mind thus reversed. Though 

 nothing is absolutely correct, everything is relatively correct in 

 direction, and we have no guide to teach us that such a reversal 

 has taken place. 



The fact is that in this, as in the other case, we involuntarily 

 take our body as the measuring rod of the universe. Distance is 

 at first estimated by the length of the arm, dimension by the 

 spread of the fingers and the sweep of the arm, and direction by 



