159. It was upon the extension of the metaphor of impression to sight as well as touch, and the consequent notion that body, with its inseparable qualities, revealed itself through both senses, that Berkeley first fastened. Is it evident, as Locke supposed it to be, that men ‘perceive by their sight’ not colours merely, but ‘a distance between bodies of different colours and between parts of the same body’; [1] in other words, situation and magnitude? To show that they do not is the purpose of Berkeley’s ‘Essay towards a new Theory of Vision.’ He starts from two principles which he takes as recognised: one, that the ‘proper and immediate object of sight is colour’; the other, that distance from the eye, or distance in the line of vision, is not immediately seen. If, then, situation and magnitude are ‘properly and immediately’ seen, they must be qualities of colour. Now in one sense, according to Berkeley, they are so: in other words, there is such a thing as _visible_ extension. We see lights and colours in ‘sundry situations’ as well as ‘in degrees of faintness and clearness, confusion and distinctness.’ (_Theory of Vision_, sec. 77.) We also see objects as made up of certain ‘quantities of coloured points,’ _i.e._ as having visible magnitude. (Ibid. sec. 54.) But situation and magnitude _as visible_ are not external, not ‘qualities of body,’ nor do they represent by any _necessary_ connection the situation and magnitude that are truly qualities of body, the mind, ‘without the mind and at a distance.’ These are tangible. Distance in all its forms--as distance from the eye; as distance between parts of the same body, or magnitude; and as distance of body from body, or situation--is tangible. What a man means when he says that ‘he sees this or that thing at a distance’ is that ‘what he sees suggests to his understanding that after having passed a certain distance, to be measured by the motion of his body which is perceivable by touch, he shall come to perceive such and such tangible ideas which have been usually connected with such and such visible ideas’ (Ibid. sec. 45). On the same principle we are said to see the magnitude and situation of bodies. Owing to long experience of the connection of these tangible ideas with visible ones, the magnitude of the latter and their degrees of faintness and clearness, of confusion and distinctness, enable us to form a ‘sudden and true’ estimate of the magnitude of the former (_i.e._ of bodies); even as visible situation enables us to form a like estimate of the ‘situation of things outward and tangible’ (Ibid. secs. 56 and 99). The connection, however, between the two sets of ideas, Berkeley insists, is habitual only, not necessary. As Hume afterwards said of the relation of cause and effect, it is not constituted by the nature of the ideas related. [2] The visible ideas, that as a matter of fact ‘suggest to us the various magnitudes of external objects before we