What is the Unity of Consciousness ?

At any given time, a subject has a multiplicity of conscious experiences. A subject might simultaneously have visual experiences of a red book and a green tree, auditory experiences of birds singing, bodily sensations of a faint hunger and a sharp pain in the shoulder, the emotional experience of a certain melancholy, while having a stream of conscious thoughts about the nature of reality. These experiences are distinct from each other: a subject could experience the red book without the singing birds, and could experience the singing birds without the red book. But at the same time, the experiences seem to be tied together in a deep way. They seem to be unified, by being aspects of a single encompassing state of consciousness. This is a rough characterization of the unity of consciousness. There is some intuitive appeal to the idea that consciousness is unified, and to the idea that it must be unified. But as soon as the issue is raised, a number of questions immediately arise. (1) What is the unity of consciousness? What does it mean to say that different states of consciousness are unified with each other, or that they are part of a single encompassing state? The idea of unity is multifaceted, and has been understood in many different ways by different thinkers. In some senses of “unity”, the claim that consciousness is unified may be obvious or trivial. In other senses, the claim may be obviously false. So the first project in this area is to distinguish between varieties of unity, and to isolate those varieties that pose the most important questions. (2) Is consciousness necessarily unified? Some thinkers (Descartes and Kant, for example) have argued that some sort of unity is a deep and essential feature of consciousness. On this view, the conscious states of a subject are necessarily unified: it is impossible for