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ence that is difficult to explore by the objective means that illuminate the brain. (In animals less developed than humans, consciousnessmight be limited to the ability to sense stimuli and respond directly to them. My use of the term goes beyond that baseline level to include human thinking, feeling, and self-awareness.)

The subjectivity of internal experience leads to what philoso- phers call the problem of other minds: in principle, we can never truly grasp the nature of anyone elses inner life. In this view, though it is a chilling thought, we cannot be sure that other people have inner lives at all.They might be zombiesbehaving like humans, but lack- ing internal experiences, including emotions and feelings. Regardless of this philosophical point, of course, we all go through life assuming that other people are much like us inside, but the idea of zombies is less far-fetched than we might think. Brain injuries can cause the loss of certain emotional reactions, and psychiatric practice recognizes zombie-like characteristics in some people, who are known as socio- paths. Their actions seem to be appropriate expressions of normal feelings, but they are only playacting, because inside they are devoid of compassion or empathy for others.

The problem of other minds illustrates the difficulty of unravel- ing consciousness by scientific means. As the neurologist Antonio Damasio puts it:How can science approach interior phenomena that can be made available only to a single observer and hence are hope- lessly subjective?But now, it seems that brain activity can be made widely observable and linked to interior states through such means as PET and fMRI.We can begin to deeply explore what has been called the last frontier of neuroscience, and the philosopher John Searle, of the University of California, Berkeley, declares to be the most im- portant problem in the biological sciences; namely,How exactly do neurobiological process in the brain cause consciousness?This ques- tion is equally important for the cognitive science of humans, and of artificial beings.

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