ABSTRACT
In a later work, the Dissertation: Exhibiting the Progress of Metaphysical, Ethical, and Political Philosophy, since the Revival of Letters in Europe (1815), Stewart seems to establish an even stronger connection between sensations and extension.23 The discussion of the relation between sensations of color and visible figure occurs in a passage where Stewart discusses the role of signs in sensory perception. Signs in sensory perception escape our attention because we immediately pass on to the things signified by them. We perceive distance by sight by comparing a variety of signs, but, just a moment later, we can hardly recollect any step of this process. In the same manner, we do not pay attention to color sensations. These modifications of our mind were intended by nature to be just signs ‘indicating to us the figures and distances of things external’ (Stewart 1854, 132). If we perceived just one color, without variety of light and shade, the organ of sight would be useless. Indeed ‘it is by the varieties of colour alone that the outlines or visible figures of bodies are so defined, as to be distinguishable one from another’ (Stewart 1854, 132). Moreover, the variety of colors is also essential for the perception of distance. The various signs by which we judge of distance by sight ‘presuppose the antecedent recognition of the bodies around us, as separate objects of perception’ (Stewart 1854, 132). Stewart probably thought that the various clues for distance have to be applied to an object distinguishable from others in our visual field. We can distinguish an object from others in our two-dimensional visual field, only if there is some variety of colors. This variety allows me to separate different patches of colors in my visual field and consider each one of them as representative of a distinct three-dimensional object at a distance.24 But the identification, preliminary to distance perception, of twodimensional shapes in our visual field is after all just the identification of the visible figures of objects. So the primary function of the variety of colors is really to signify the visible figure of an object. Speaking of the cataract case in his manuscripts, Reid seemed to imply that a variety of colors is a necessary condition for perceiving visible figure. In the Inquiry, however, the cataract case is not so much used to show that without variety of colors there is no perception of visible figure, but rather to make the point that the perception of color can be
G.B. Grandi60
