he Treatise on Systems of 1749, which is devoted to critiquing the metaphysics and the methodology of earlier philosophers, the Treatise on Animals of 1755, which attempts to explain how human souls differ from those of animals with respect to intelligence and free will, the “Dissertation on Liberty”, an important short work appended to the Treatise on Sensations that further addresses the issue of human free will, the “Summary [extrait raisonné] of the Treatise on Sensations,” originally included in the Treatise on Animals, and a textbook on Logic (1780). He also published a work on commerce and government, assembled a dictionary of synonyms, and put together a multi-volume course of studies that he had developed while tutoring the Prince of Parma. The latter dealt principally with history but also included some philosophical material, including a different presentation of logic. At the time of his death he left an incomplete a work entitled The Language of Calculation.
Condillac's account of the development of our cognitive capacities was informed by a particular conception of the nature of the mind and the sensations it is originally given. He argued that the mind must be an unextended or immaterial substance (Essay I.i.1 §6). When it senses, nothing actually passes into it from the outside world or the body. Instead the action of external objects on the sense organs brings about changes in the body and these changes serve as the merely occasional cause of the production of sensations in the mind.
Sensations are modifications of our being. To understand them as images of something distinct from us is to treat them as “ideas” rather than simply as “sensations,” and that is an operation that is so far from being automatic that it exceeds the capacities of animals. However, unlike Reid, who was later to argue for a rigorous distinction between sensations, considered as states of feeling experienced by the mind, and perceptions, considered as acts of thinking something about an object, Condillac maintained that sensations do lend themselves to being treated as ideas. He denied that sensations are “something that only occurs aside from thoughts and modifies them” and insisted that they are “as representative” as any other thought experienced by the mind (Essay I.ii.2 §9).
Most early modern philosophers were impressed by the facts of geometrical optics, which teach that light imprints an inverted, left-right reversed image of the external world on the concave surface of the back of the eye. If we accept that the eye is the means of visual perception, then this teaching appears to imply that the eyes filter out information about the distance at which objects are set outwards from us and transmit just information about position along the horizon and the azimuth to the mind. After all, while points at different angles of inclination upwards or downwards from the horizon, or at different compass directions will project light to different parts of the retina, points that differ only with reference to their distance outwards from the eye will project light onto the same part of the eye. But if differences in distance outwards make no difference to the impression on the eye, and the mind is only affected as a consequence of how the eye is affected, then information about outward distances is not conveyed to the mind. It would seem, therefore, that visual perception must originally lack information about distance outwards. It must consist of an awareness of images that are only two-dimensional projections of solid objects in a three dimensional space.
But the Condillac of the Essay was not impressed by these considerations. Though he admitted that the image imprinted by light on the eye is merely two-dimensional, he denied that the mind must therefore only be aware of a two-dimensional image (Essay I.vi §2). However he had little to say about how the mind might acquire information about outward distances, and instead confined himself to attacking the standard account. On the standard account, when I perceive a three dimensional obj