M.Matthen & D.Stokes, Perception and its modalities, Oxford: OUP. Beyond vision: The vertical integration of sensory substitution devices

The ability of blind people to navigate through unfamiliar environments cannot but make us wonder. It also leads to philosophical problems, which Descartes already captured: No doubt you have had the experience of walking over rough ground without a light, and fnding it necessary to use a stick in order to guide yourself. You may then have been able to notice that by means of this stick, you could feel the various objects situated around you, and that you could even tell whether there were trees or rocks, or sand, or water, or grass, or mud, or any other such thing. It is true that this kind of sensation is somewhat confused and obscure in those who do not have long practice with it. But consider it in those born blind, who have made use of it all their lives: with them, you will fnd it is so perfect and so exact, that one might almost say that they see with their hands, or that their stick is the organ of some sixth sense given to them in place of sight (Descartes, 1637/1985, p. 153).