John McDowell proposes conceptualism of perceptual content to warrant the idea that perceptual experience rationally constrains belief. To support this idea, McDowell claims that not only belief and perception, but also the world itself has a propositional structure. This view of the “unboundedness of the conceptual” is, however, doubtful. In this paper, I shall explore how we can defend conceptualism without accepting the above view. To do this, I shall propose a mechanism that gives a propositional structure to perception before it is established as a conscious experience. In so doing, I shall employ two empirical theories: the visual index theory and the sensory classification theory. This inquiry aims at revising conceptualism and giving it an empirical basis.