By far the most important of his works is the "Traité des sensations", in which Condillac treats psychology in his own characteristic way. He questioned Locke's doctrine that the senses give us intuitive knowledge of objects, that the eye, for example, naturally judges shapes, sizes, positions, and distances. He believed it was necessary to study the senses separately, to distinguish precisely what ideas are owed to each sense, to observe how the senses are trained, and how one sense aids another. He believed that the conclusion has to be that all human faculty and knowledge are transformed sensation only, to the exclusion of any other principle, such as reflection.