478 Dr Alison on Single and Correct Vision, by means of 



part of that image must be referred to the upper part of the ex- 

 ternal object corresponding to it, and vice versa; and again, 

 images formed on corresponding parts of the two retinse, and 

 referred, according to this law, to external space, must appear to 

 come from the same points, i. e. to represent the same object. 



But here the question immediately presents itself, How is this 

 law of Visible Direction originally formed in, or impressed upon, 

 the mind ? If it be thought to be acquired by experience and 

 association, the observations already made apply to, and I think 

 set aside that supposition. If it be thought to be independent of 

 experience, it implies, in the first place, that the mind has an ori- 

 ginal perception of distance by the eye, i. e. that it is originally 

 aware of impressions on the retina being produced by causes at 

 a distance from the body, although it draws no such inference 

 from impressions on the skin ; whereas not only Berkeley and his 

 followers, but Re id, and most other authors on this subject, have 

 believed that it is only by experience that we learn that visible 

 objects do not touch the eye ; and this has been generally thought 

 to be supported by observations on persons to whom the sense 

 of sight has been given suddenly, and at a mature age, by couch- 

 ing. 



But farther, this doctrine implies that the mind naturally 

 draws an inference, not only as to the position of any impression 

 that may be made on the retina, but as to the direction in which the 

 ray of light came, that made that impression ; and as that direction 

 is not a direct object of sense, and as I apprehend that no reason 

 can be given, why a ray should be supposed to have come in the 

 direction of a perpendicular to the surface of the retina, rather 

 than of any other line falling on that surface, — this theory really 

 ascribes our perception of the true number and position of ob- 

 jects by the eye merely to the principle of Intuition, i. e. it merely 

 states the fact, that the images formed on the retina are referred 

 to places in the external world according to this law ; and if we 

 regard the theory as a sufficient explanation, we must regard this 

 as an ultimate fact in our mental constitution. 



