"Consciousness. The having of perceptions, thoughts, and feelings; awareness....When asked what consciousness is, we have no better answer than Louis Armstrong's when a reporter asked him what jazz is: "Lady, if you have to ask, you'll never know." (quoted in Pinker, How the Mind Works, p.60) 

"The term is impossible to define except in terms that are unintelligible without a grasp of what consciousness means. Many fall into the trap of equating consciousness with self-consciousness -- to be conscious it is only necessary to be aware of the external world. Consciousness is a fascinating but elusive phenomenon; it is impossible to specify what it is, what it does, or why it evolved. Nothing worth reading has been written on it." 
Stuart Southerland, The International Dictionary of Psychology

There are many forms of consciousness, such as those associated with vision, thinking, emotion, pain, and so on, including self-consciousness. Consciousness (or rather, self-consciousness) has traditionally been used as a defining characteristic of humans as opposed to animals. Descartes used the Cogito as an act of self-definition, and considered animals as machines. Descartes' followers in Port Royal are said to have tortured animals with the confident conviction that their cries of agony were comparable to the noises from machinery. In a note to the Systema naturae, Linneaus dismissed the Cartesian theory that conceived of animals as if they were automata mechanica with the vexed statement: "Surely Descartes never saw an ape." (Giorgio Agamben, The Open, p. 23) Today, it seems increasingly difficult to deny animals any form of consciousness. 

It can be useful to distinguish between "perceptual consciousness" and "reflective consciousness." 

Perceptual consciousness is "the state or faculty of being mentally conscious or aware of anything." (OED) It is a basic conception of consciousness as being aware of something, and it may entail memories, anticipations, or thinking about nonexistent objects or events as well as immediate sensory input. (see Donald R. Griffin, Animal Minds p.10) Gerald Edelman calls this form of consciousness "primary consciousness," which is seen in animals who are able to construct a "mental scene" but have limited semantic or symbolic capabilities and no true language. (A Universe of Consiousness, Chapt 9) 

For Edelman, primary consciousness requires perceptual organization, the ability to carve up the world of signals into categories useful for a given species. (cf frame problem) It requires concepts -- the ability to combine different perceptual categorizations and to abstract some common features -- as well as memory and value (to develop categorical responses that are adaptive.) Following William James, who described the "specious present" in which perceptual and memory systems interact, Edelman calls the ability to construct a consious scene "the remembered present." For Edelman, the emergence of primary consciousness gives a significant selective advantage -- the ability of an animal to evade complex dangers by selecting its responses to a complex environment through the construction of a complex scene based on its own unique history of value-dependent responses. " (p.109) For Edelman, Identitymemory, and space compose primary consciousness. From this primary consciousness, a higher-order consciousness evolves in man, with the powers of language, conception, and thought. 

Reflective (or self-) consciousness is "the recognition by the thinking subject of its own acts or affections." (OED) (see mirror ) This recognition requires an internalization of otherness. (see also desire ) 

In order to develop a theory of consciousness, is it also necessary to posit an unconscious ? According to Freud, the ego is an agency of the psyche,by means of which the subject aquires a sense of unity and identity, "a coherent organization of mental processes." (XIX,17.) Through consciousness, the ego is the site of differentiation between inside and outside, between "subjective" and "objective." Freud's central idea -- that conscious recollections are inevitably distorted by a person's wishes, desires, and unconscious conflicts -- became a core assumption of all psychoanalysis. 

If we "explained" consciousness, what would count as an explanation? What kind of theory would be required to make the relation between matter and mind "transparent?" (see mind / brain 

According to Daniel Dennett, our powerful subjective impression that we are conscious of sensory perceptions in real time is an illusion. (but isn't this the illusion we live by?) This is know as the problem of qualia . 

Is introspection able to provide a theory of consciousness? (see thinking) Is there a difference between a scientific account and one that would be personally satisfactory? "it is natural that I demand an explanation of my own consciousness in terms satisfactory to my self. But I must realize that it is not a scientific act to do so." (Edelman, p.138) "There is no more mystery to our inability as scientists to give an explanation of individual consciousness than there is to our inability to explain why there is something rather than nothing."

Henri Bergson downplays the role of representation in perception in favor of movement and radius of action. For him, the nervous system correlates points in space with motor mechanisms, in a strict law which he formulates thus: "perception is the master of space in the exact measure in which action is master of time." (Matter and Memory, p.32) On the other hand, for Bergson, memory is the intersection of mind and matter. Contemporary views of consciousness think of it as a dynamic integration of past, present, and self, and that memory is part of its dynamics. (see also sentience

Does it make sense to speak of consciousness without the body ?

Neuroscientists believe that examining neurons and the interactions between them could yield the necessary empirical, unambiguous knowledge for the construction of scientific models of consciousness. But it is not clear whether the kind of "electrophysiological" theory called for by Francis Crick would suffice to explain consciousness. Crick and Christof Koch argue in The Astonishing Hypothesis that consciousness is the combination of attention and short-term memory

One of the most contested issues in neurology concerns the localization of behaviour, or memory. Oliver Sachs caricatures the two opposing schools of thought as "splitters" and "lumpers." While the majority of neurologists favor "parcellation" of the brain, others felt that there must be higher order functions (such as memory, attention, emotion, thought, consciousness, identity) which required large-scale or global processes in the brain. 

Oliver Sachs invokes Gerald Edelman's " bottom-up" theories of consciousness which looks to the biological orgins of consciousness in the perceptual processes and their mappings in the organism. Alterations of local mapping are sufficient cuases for alterations of consciousness. It is not necessary to invoke any additional cause, such as a coexisting "top-down" neurosis, like hysteria, or psychosis. 

"The Binding Problem" is the issue raised by the non- local interactions of neurons. How can a set of diverse and functionally segregated maps cohere without a higher-order controller? Even a single scene is processed by different parts of the brain. Are oscillation frequencies or synchronies the ties that bind? Both chaos and complexity theory offer ways of addressing the firings of large groups of neurons. Gerald Edelman claims to have "solved" consciousness as a process of neural Darwinism, in which groups of neurons compete to form an effective representation of the world. He calls this the theory of neuronal group selection. (TNGS) 

Another group, dubbed the "new mysterians" argues that materialist explanations are fundamentally inadequate, or that humans will never be able to understand their own consciousness, or, as Roger Penrose argued, that mental activity is linked to quantum behaviour that is affected by observation. 

Consciousness is also an object of historical inquiry. Each epoch is understood to have its own particular forms and contents of (collective) consciousness. Historians of technology also describe transformations of "technological consciousness." Social groups and classes are understood to have their own distinctive consiousness. (see also ideology )