From the experimental point of view, the art of knowing demands skill in selecting appropriate sense-data on one side and connecting principles, or conceptual theories, on the other. It requires adeveloped and constantly progressive technique to settle upon both the observational data and the idea that assist inquiry in reaching aconclusion in any particular case. But in Kant's view, the distinction and the connection between the two, while necessary to anything which may be termed cognition, have nothing to do with the validity of any particu- lar enterprise of knowing. Illusion and error exemplify the synthesis of sense and understanding quite as much as does the soundest instance of scientific discovery. In one case, the heart of the whole matter is the exercise of adifferential control which makes the difference between good and bad knowing. In Kant's scheme the blessings of the categories descend upon the material of sense without reference to making adistinction be- tween the true and the false.