The fact that distance or outness is invisible is sometimes regarded as Berkeley’s contribution to the theory of seeing. It is rather the assumption on which the _Essay_ proceeds (sect. 2). The _Essay_ does not prove this invisibility, but seeks to shew how, notwithstanding, we learn to find outness through seeing. That the relation between the visual signs of outness, on the one hand, and the real distance which they signify, on the other, is in all cases arbitrary, and discovered through experience, is the burden of sect. 2-40. The previously recognised signs of “considerably remote” distances, are mentioned (sect. 3). But _near_ distance was supposed to be inferred by a visual geometry—and to be “suggested,” not signified by arbitrary signs. The determination of the visual signs which suggest outness, near and remote, is Berkeley’s professed discovery regarding vision. An induction of the visual signs which “suggest” distance, is followed (sect. 43) by an assertion of the wholly sensuous reality of _colour_, which is acknowledged to be the only immediate object of sight. Hence _visible_ extension, consisting in colour, must be dependent for its realisation upon sentient or percipient mind. It is then argued (sect. 44) that this mind-dependent visible outness has no resemblance to the tangible reality (sect. 45). This is the first passage in the _Essay_ in which Touch and its data are formally brought into view. Tactual or locomotive experience, it is implied, is needed to infuse true reality into our conceptions of distance or outness. This cannot be got from seeing any more than from hearing, or tasting, or smelling. It is as impossible to see and touch the same object as it is to hear and touch the same object. Visible objects and ocular sensations can only be _ideal signs_ of _real things_. The sections in which Touch is thus introduced are among the most important in the _Essay_. They represent the outness given in hearing as wholly sensuous, ideal, or mind-dependent: they recognise as more truly real that got by contact and locomotion. But if this is all that man can see, it follows that his _visible_ world, at any rate, becomes real only in and through percipient mind. The problem of an _Essay on Vision_ is thus, to explain _how_ the visible world of extended colour can inform us of tangible realities, which it does not in the least resemble, and with which it has no _necessary_ connexion. That visible phenomena, or else certain organic sensations involved in seeing (sect. 3, 16, 21, 27), gradually _suggest_ the real or tangible outness with which they are connected in the divinely constituted system of nature, is the explanation which now begins to dawn upon us.