Thinking about consciousness is no longer the preserve of philosophers; it is part of science too. Experts from a range of disciplines give their individual views

The problem of consciousness is to say what it is for some of our thoughts, feelings, and sensations to be conscious, given that others are not. This is different from saying what it is for a person to be conscious or not conscious. Even when people are conscious, many of their thoughts and sensations typically are not. And there is nothing problematic about a person being conscious - it is just the person being awake and responsive to sensory input.

When a thought or feeling is conscious, one is always conscious of that thought or feeling. Being conscious of things means either sensing them or having thoughts about them. And the thought or sensation that makes us conscious of something need not itself be conscious; even sensing subliminally makes us conscious of things in a certain way.

We do not sense our conscious thoughts and sensations - there is no sense organ for doing so. The only alternative is that we are conscious of our conscious thoughts, feelings, and sensations by having thoughts about them. These "higher order thoughts" are themselves seldom conscious; so we are unaware of them.

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This model explains many striking phenomena. For example, creatures that can talk about thoughts and sensations at all can readily describe their own conscious thoughts and sensations, but not those that are not conscious. The model predicts this. When one says that one has a thought or sensation, one expresses a thought about that thought or sensation; indeed, one expresses the very higher order thought in virtue of which one is conscious of that thought or sensation. Similarly, we cannot report thoughts and sensations that are not conscious because then there is no higher order thought for us to express.

The absence of higher order thoughts prevents us from responding to our experiences verbally, but does not preclude non-verbal responses. This helps explain what happens in disorders such as blindsight, where brain lesions prevent subjects from consciously experiencing visual input in certain areas of their visual field. These patients cannot verbally report about that input in the automatic way that is characteristic with conscious experiences, but they can respond non-verbally to such input. These help confirm the higher-order-thought model, and make it promising for future research.

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David Rosenthal is professor of philosophy, City University of New York.