 Finally, we might attempt to determine whether any unified account exists that applies generally to all of the perceptual modalities. We can ask this question either at the level of quite specific claims, such as those concerning the objects of perception or the nature and structure of content. We can ask it about the relationships among perceiving, believing, and acting. Or we can ask it about the general theory of necessary and sufficient conditions for perceiving. Some philosophers, impressed by findings concerning non-visual modalities, express skepticism whether a unified theory exists (e.g., Martin 1992).