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tual categories, and through the process of reentry,constantly trade information back and forth as if the brain were talking to itself.

Edelman andTononi conclude that interactions between two par- ticular structures in the brain are mostly responsible for consciousness: the cortex or gray matterthe outer layer of neurons that deals with sensory impulses and higher mental functionsand the thalamusa part of the brain associated with emotion. Moreover, there are two levels of consciousness. Primary consciousness is perceptual awareness of the world in the present, but it is not consciousness of self. That level comes with higher-order consciousness, which depends on lan- guage and on social interactions and which has knowledge about the past and future as well as the present; it is what humans add to their primary consciousness.

The physician and historian of ideas Israel Rosenfeld also believes in the importance of coherence over time, the sense of self we main- tain as a continuous internal presence throughout our lives, or at least our adult lives. As William James saw more than a century ago, this long-term coherence is a function of memory, and Rosenfield em- phasizes that consciousness and memory are in a certain sense inseparable, and understanding one requires understanding the other.But how does this continuous memory develop? According to Rosenfield, memory is created because the brain resides in a body:

My memory emerges from the relation between my body . . . and my brains imageof my body (an unconscious activity in which the brain creates a constantly changing generalized idea of the body . . . ). It is this relation that creates a sense of self.

None of these approaches is a definitive explanation of con- sciousness that is supported by complete scientific evidence. It can be argued also that none truly confronts the hard problem of subjective experience and why we have it, at least not within the framework of what we know about the brain. Edelman and Tonioni touch on this issue when they write,

while we can construct a sensible scientific theory of consciousness . . . that theory cannot replace experience: Being is not describing.A scientific explanation can have predictive and explanatory power, but it cannot di-

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