Dr A.ison 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 @ 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 imverted, 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 light reflected from a single ob- 
