1. INTRODUCTIONIt is ten to noon and I am sitting in a park, leafing through a catalogue of Mondrian's paintings. While I look at the photograph of Composition with Red, Blue and Yellow, I peripherally see the text describing the painting, my hands holding the book, and, beyond the book, some people and trees, and a fountain in the background. At the same time, I hear the fountain's water pouring down, girls chatting on my left, leaves rustling and children laughing, and other background noises. I have my legs crossed, and I am aware of that. Further, I am eating nut chocolate, tasting its flavor and feeling the little nut pieces in my mouth, and smelling the pleasant odor of the roses behind me. Unfortunately, I have a bad headache, so I am thinking that I should take some aspirin. All those conscious states occur simultaneously at ten to noon, and they belong to the same subject-me. They occur together, and I experience them as occurring together.The conscious states I have at ten to noon also seem to be tightly related to each other. The picture of Composition appears to me as a unified image. Moreover, it appears to me as being part of the book I hold in my hands. It also appears as standing in a specific spatial relation with the text which describes it (say, on its left). The book itself appears to me as a unified thing, and as bearing spatial relations with me and the other objects in my visual field-for example, it is closer to me than the trees and the fountain in the background. The objects around me are unitarily perceived by me through different sense modalities-the fountain, for instance, is both heard and seen. My thought about aspirin is causally related to my feeling pain in my head. And so on. Not only all the conscious states I have at ten to noon appear to me as occurring together. They also appear as being intimately connected in such a way to form a unified state of consciousness.At any one time we simultaneously have a number of conscious states. It seems intuitively true that such conscious states are somehow unified. But what makes them unified? In virtue of what are all the conscious states one has at any given time unified? This is the problem of the unity of consciousness. Before attempting any solution to the problem, it is worth drawing some distinctions, and to clarify what exactly the phenomenon at issue is.First, what we are concerned with is phenomenal unity, namely the unity of one's phenomenally conscious states. This must be distinguished from other kinds of unity, such as access unity (the unity of one's access-conscious states), introspective unity (the unity of one's introspected conscious states), and cognitive unity (the integration and coordinated use of one's mental faculties, aimed at task accomplishment, problem solving, and interaction with the environment).1 It must also be distinguished from behavioral unity (the capacity to coordinate one's body parts in order to produce more or less complex actions), and neurophysiological unity (the unity of a set of conscious states which are correlated to or brought about by the same area of the brain).2Second, unity of consciousness is unity of all the phenomenally conscious states of a subject, rather than a subset of them. Thus, it must be distinguished from both objectual and spatial unity, which are only forms of partial unity. Objectual unity is the kind of unity which binds together, say, your perceiving redness and your perceiving roundness, thereby enabling you to perceive a red ball as a unified object. It also enables you to perceive one and the same object through different sense modalities. Objectual unity is only partial unity because, usually, not all the conscious states one has at a time are directed to one and the same object. Spatial unity is the unity of conscious states which represent objects as belonging to the same space. It is merely partial because not all the conscious states one has at a time represent objects as being located in space, or as bearing spatial relations with other objects. …
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