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THE PROGRESS OF PHILOSOPHY has often been determined by the discovery of paradoxes and the formulation of successive, improved attempts to resolve them. The paradoxical conclusions uncovered by Parmenides and Zeno are an outstanding example. However, these paradoxes can sometimes go unnoticed. They are obscured because philosophers will often try to avoid them from the outset, so that only the route around the difficulty is presented to the public, not the obstacle that motivated the detour. My project in this paper is to expose one such hidden paradox, which I refer to as Condillac's paradox, not because Condillac discovered it, but because he was less successful at avoiding it than were his contemporaries.
Condillac's paradox arises from two innovations introduced into the philosophy of the early modern period. One was the rejection of the Aristotelian view that sensation is a physiological operation carried out by material sense organs, which take on the form of external objects, and its replacement by the view, paradigmatically articulated by Descartes, that sensations are modifications or modes of thought of an immaterial mind. The second was the rejection of the view that colors are qualities of external objects, and its replacement by the view that colors are merely sensations. Taken together with the facts, apparent from visual perception, that colors are extended and have shape, or that if they are not it is at least the case that many colored points are simultaneously presented alongside one another on a visual field to constitute extended, colored shapes, these innovations give rise to the following four mutually antagonistic propositions:
i) Sensations are modifications of the mind,
ii) The mind is unextended.
iii) Colors are sensations,
iv) Colors are extended.
These four propositions are not patently irreconcilable. On the contrary, it might seem that one or two careful distinctions are all that would be required to remove any initial appearance of incompatibility between them. For instance, we might reject the assumption that the mind has the characteristics that its modifications have. Or, if this seems too abrupt (because not to have the characteristic that a modification has just means not to be modified by that characteristic, and so just means not to have that particular modification), we might say that what modifies the...