Dr Alison on Single and Correct Vision. 473 



connected, or by which it is suggested ; and, in a case where the 

 very same notions seem to be suggested by different sensations, 

 we may expect to arrive at the knowledge of the manner, in which 

 the intimations acquired by the different senses are made to cor- 

 respond. 



When the attention is fixed for the first time on the fact, that, 

 although the notions which we acquire of the number and posi- 

 tion of external objects, during the ordinary exercise of the sense 

 of sight, are correct, yet the images on the retina?, which are the 

 essential conditions of our seeing any one of these objects, are 

 double, inverted, and reverted, — the natural inference certainly is, 

 that some explanation should be sought, and may be had, of so 

 singular an anomaly. But a little reflection will shew, that our 

 notions of the number and position of objects are not connected 

 merely with the exercise of the sense of Sight, but very much 

 with that of Touch. And when we find it stated by many phi- 

 losophers, that we have no notions on those points naturally and 

 originally connected with Sight, and that it is only by experi- 

 ence, and by association with the notions acquired by Touch, that 

 we learn to form judgments of the number and position of ob- 

 jects by the eye — we must admit, that it is only by appeal to 

 facts, not by any reasoning a priori, that the truth or falsehood 

 of this doctrine can be determined. We must therefore satisfy 

 ourselves, that number and position are original, not acquired, 

 perceptions of the eye, before we are entitled to ask for an ex- 

 planation of single and correct vision, by double and inverted 

 images. 



The late Dr Brown was so confident of the perception of the 

 number and position of visible objects being acquired only by as- 

 sociation or custom, that he thought himself justified in dis- 

 missing the subject with the following observations : " In the 

 single vision of the erect object, from a double image of the ob- 

 ject inverted, there is nothing at all mysterious to any one who 

 has learnt to consider, how much of the visual perception is 

 referable to association. If the fight reflected from a single ob- 



