C as coming in the straight line cc from the lower end of the object; and the stroke or impulse on a as coming in the line AA from the upper end of the object, is directed to make a right judgment of the situation of the object ABC, notwithstanding the picture of it is inverted.  This is illustrated by conceiving a blind man who, holding in his hands two sticks that cross each other, doth with them touch the extremities of an object, placed in a perpendicular situation.  It is certain this man will judge that to be the upper part of the object which he touches with the stick held in the undermost hand, and that to be the lower part of the object which he touches with the stick in his uppermost hand.  This is the common explication of the erect appearance of objects, which is generally received and acquiesced in, being (as Mr. Molyneux tells us [Diopt. par. 2. c. 7.  P. 289.]) ’allowed by all men as satisfactory’.

90.  But this account to me does not seem in any degree true.  Did I perceive those impulses, decussations, and directions of the rays of light in like manner as hath been set forth, then indeed it would not be altogether void of probability.  And there might be some pretence for the comparison of the blind man and his cross sticks.  But the case is far otherwise.  I know very well that I perceive no such thing.  And of consequence I cannot thereby make an estimate of the situation of objects.  I appeal to anyone’s experience, whether he be conscious to himself that he thinks on the intersection made by the radious [Sic] pencils, or pursues the impulses they give in right lines, whenever he perceives by sight the position of any object?  To me it seems evident that crossing and tracing of the rays is never thought on by children, idiots, or in truth by any other, save only those who have applied themselves to the study of optics.  And for the mind to judge of the situation of objects by those things without perceiving them, or to perceive them without knowing it, is equally beyond my comprehension.  Add to this that the explaining the manner of vision by the example of cross sticks and hunting for the object along the axes of the radious pencils, doth suppose the proper objects of sight to be perceived at a distance from us, contrary to what hath been demonstrated.

91.  It remains, therefore, that we look for some other explication of this difficulty:  and I believe it not impossible to find one, provided we examine it to the bottom, and carefully distinguish between the ideas of sight and touch; which cannot be too oft inculcated in treating of vision:  but more especially throughout the consideration of this affair we ought to carry that distinction in our thoughts:  for that from want of a right understanding thereof the difficulty of explaining erect vision seems chiefly to arise.