13.  Since, therefore, those angles and lines are not themselves perceived by sight, it follows from sect. 10 that the mind doth not by them judge of the distance of objects.

14.  Secondly, the truth of this assertion will be yet farther evident to anyone that considers those lines and angles have no real existence in nature, being only an hypothesis framed by the mathematicians, and by them introduced into optics, that they might treat of that science in a geometrical way.

15.  The third and last reason I shall give for rejecting that doctrine is, that though we should grant the real existence of those optic angles, etc., and that it was possible for the mind to perceive them, yet these principles would not be found sufficient to explain the phenomena of distance, as shall be shown hereafter.

16.  Now, it being already shown that distance is suggested to the mind by the mediation of some other idea which is itself perceived in the act of seeing, it remains that we inquire what ideas or sensations there be that attend vision, unto which we may suppose the ideas of distance are connected, and by which they are introduced into the mind.  And first, it is certain by experience that when we look at a near object with both eyes, according as it approaches or recedes from us, we alter the disposition of our eyes, by lessening or widening the interval between the pupils.  This disposition or turn of the eyes is attended with a sensation, which seems to me to be that which in this case brings the idea of greater or lesser distance into the mind.

17.  Not that there is any natural or necessary connection between the sensation we perceive by the turn of the eyes and greater or lesser distance, but because the mind has by constant experience found the different sensations corresponding to the different dispositions of the eyes to be attended each with a different degree of distance in the object:  there has grown an habitual or customary connection between those two sorts of ideas, so that the mind no sooner perceives the sensation arising from the different turn it gives the eyes, In order to bring the pupils nearer or farther asunder, but it withal perceives the different idea of distance which was wont to be connected with that sensation; just as upon hearing a certain sound, the idea is immediately suggested to the understanding which custom had united with it.

18 Nor do I see how I can easily be mistaken in this matter.  I know evidently that distance is not perceived of itself.  That by consequence it must be perceived by means of some other idea which is immediately perceived, and varies with the different degrees of distance.  I know also that the sensation arising from the turn of the eyes is of itself immediately perceived, and various degrees thereof are connected with different distances, which never fail to accompany them into my mind, when I view an object distinctly with both eyes, whose distance is so small that in respect of it the interval between the eyes has any considerable magnitude.