19.  I know it is a received opinion that by altering the disposition of the eyes the mind perceives whether the angle of the optic axes is made greater or lesser.  And that accordingly by a kind of natural geometry it judges the point of their intersection to be nearer or farther off.  But that this is not true I am convinced by my own experience, since I am not conscious that I make any such use of the perception I have by the turn of my eyes.  And for me to make those judgments, and draw those conclusions from it, without knowing that I do so, seems altogether incomprehensible.

20.  From all which it follows that the judgment we make of the distance of an object, viewed with both eyes, is entirely the result of experience.  If we had not constantly found certain sensations arising from the various disposition of the eyes, attended with certain degrees of distance, we should never make those sudden judgments from them concerning the distance of objects; no more than we would pretend to judge a man’s thoughts by his pronouncing words we had never heard before.

21.  Secondly, an object placed at a certain distance from the eye, to which the breadth of the pupil bears a considerable proportion, being made to approach, is seen more confusedly:  and the nearer it is brought the more confused appearance it makes.  And this being found constantly to be so, there ariseth in the mind an habitual connection between the several degrees of confusion and distance; the greater confusion still implying the lesser distance, and the lesser confusion the greater distance of the object.

22.  This confused appearance of the object doth therefore seem to be the medium whereby the mind judgeth of distance in those cases wherein the most approved writers of optics will have it judge by the different divergency with which the rays flowing from the radiating point fall on the pupil.  No man, I believe, will pretend to see or feel those imaginary angles that the rays are supposed to form according to their various inclinations on his eye.  But he cannot choose seeing whether the object appear more or less confused.  It is therefore a manifest consequence from what bath been demonstrated, that instead of the greater or lesser divergency of the rays, the mind makes use of the greater or lesser confusedness of the appearance, thereby to determine the apparent place of an object.

23 Nor doth it avail to say there is not any necessary connection between confused vision and distance, great or small.  For I ask any man what necessary connection he sees between the redness of a blush and shame?  And yet no sooner shall he behold that colour to arise in the face of another, but it brings into his and the idea of that passion which hath been observed to accompany it.