There have been countless definitions of the word 'consciousness'. Indeed there have also been hundreds of books on consciousness. It's a debate that's hot and trendy. However, it's also often pointless. The main reason for this is that people are nearly always talking about different things when they talk about consciousness. More relevantly, they define the word 'consciousness' in very different ways. Moreover, many who talk or write about consciousness never actually get around to defining the word 'consciousness' at all. True, they may have their own tacit or unexpressed pet definitions deep within their minds; though they never explicate or articulate such definitions precisely or in any detail.
As a result of all this, perhaps it would be wise to adopt a deflationary view of the word 'consciousness'. That's what the English philosopher Kathleen Wilkes did when she wrote that
"perhaps 'consciousness' is best seen as a sort of dummy-term like 'thing', useful for the flexibility that is assured by its lack of specific content".
We can agree with Wilkes and also see the word 'consciousness' as a bundle-term. It is so because it has so many meanings, definitions and connotations.
Though if the word 'consciousness' is indeed a dummy- or bundle-term, then surely spending any time on definitions may seem a little pointless. Then again, in the case of Wilkes' other example of the word 'thing', if we can even define that word to some degree of approximation and detail, then surely we can do the same with 'consciousness'.
The psychologists James Ward and Alexander Bain (writing at the end of the 19th century and as quoted by Edward Titchener) took a strong line against the ostensible liberalism (or pluralism) of people like Kathleen Wilkes. Ward and Bain believed that it's precisely the fact that the word 'consciousness' is a dummy- or bundle-term that traps us in the mud. They wrote:
“'Consciousness' is the vaguest, most protean, and most treacherous of psychological terms.”
With words like that, one can see how it didn't take long for behaviorism to take up its hegemonic position in psychology and philosophy in the 1920s and beyond.
In addition, judging by Professor Ward's use of the word 'protean', one can also conclude that not only did he believe the word 'consciousness' to be vague, he also believed that it could be made to mean what any writer, philosopher or layperson wanted it to mean. (Think here of the “spiritual” uses of the word 'consciousness'.)
Thus, as a result of all this, William James (writing at roughly the same time as Ward), didn't offer his readers a single definition of the word 'consciousness' in his well-known book Principles of Psychology.
Thus, in 1913, John B. Watson had this to say (in his paper 'Psychology as the Behaviorist Views it'):
“The time seems to have come when psychology must discard all references to consciousness... This suggested elimination of states of consciousness as proper objects of investigation in themselves will remove the barrier from psychology which exists between it and the other sciences.”
Of course Watson wasn't that concerned with the definitions of the word 'consciousness'. Instead he had a problem with consciousness itself. That problem was its non-scientific status (or even its metaphysical reality). However, there's a connection here. Perhaps the definitions of the word 'consciousness' are both so multifarious and vague precisely because of the non-scientific (i.e., private) nature of consciousness. If consciousness were as intersubjective a phenomenon as a cat or a neuron, then we wouldn't have such many multifarious and vague definitions.
Now let's move forward to the late 20th century.
We can also say that the spirit (if not the letter) of behaviourism was revived by eliminative materialists.
On the one hand, behaviourists didn't necessarily claim that consciousness isn't a feature of the human mind; they simply believed it to be non-scientific. Some/all eliminitive materialists (or at least Patricia Churchland), on the other hand, believe that “consciousness [might] go the way of 'caloric fluid' or 'vital spirit'”. Thus some people may think that eliminitive materialists simply want to eliminate “propositional attitudes” and the “folk psychology” which has partly been built on them. However, they may want to eliminate consciousness (or at least references to 'consciousness') in the long run too.
This would also explain why definitions of 'consciousness' are so vague and multifarious. Indeed, if consciousness doesn't so much as exist (at least as it's seen by many people), then of course the definitions of it will be vague and multifarious!
What Consciousness Does/Seems
One major problem with definitions of consciousness is that many philosophers, scientists or laypersons often fixate on what may only be a single aspect (though sometimes aspects) of consciousness and then go on to more or less ignore – or even deny – the rest. This is one reason why we have a vast of amount of sexy, titillating and pseudo-scientific theories of consciousness. That is, the philosopher, scientist or layperson overplays his angle on consciousness seemingly without realising that it may simply be precisely that: a single aspect.
In extremely broad terms, this can be seen in the debate between those who say that “consciousness is what consciousness does” and those who say that “consciousness is how consciousness seems”.
For example, arguably it can be said that the philosopher David M. Rosenthal is concerned with how consciousness seems when he writes (in his paper 'A Theory of Consciousness') the following:
“Intentional and sensory properties constitute the most likely candidate [for a definition of 'consciousness']; all mental states have one or the the other.”
Of course it can be seen that there's a small fusion here. After all, doesn't the intentional have an impact on what consciousness does? Indeed the same can even be said about “sensory properties”. Don't sensory properties (or even qualia) carry information? And if that's the case, then sensory information can result in a person doing x rather than y.
Jerry Fodor has also made this point (in a review of Colin McGinn's The Problem of Consciousness) when he wrote the following:
“It used to be universally taken for granted that the problem about consciousness and the problem about intentionality are intrinsically linked: that thought is ipso facto conscious, and that consciousness is ipso facto consciousness of some or other intentional object... concentrating on intentionality and ignoring consciousness - has proved a remarkably successful research strategy so far.”
On the other hand, John Searle (in his The Rediscovery of the Mind) connects intentionality to consciousness. Thus:
"Only a being that could have conscious intentional states could have intentional states at all, and every unconscious intentional state is at least potentially conscious... [T]here's a conceptual connection between consciousness and intentionality that has the consequence that a complete theory of intentionality requires an account of consciousness.”
Rosenthal connects the dots too. He writes:
“Of course many mental phenomena, such as perceptual states and emotions, have both kinds of property [the “sensory” and “intentional”]; but other mental states exhibit only one of the two.”
As for the former position (i.e., “consciousness is what consciousness does”), this will elicit causal, functional, computational, evolutionary, neuroscientific, etc. definitions of the word 'consciousness'.
It's quite easy for the defenders of the former position to pay little - or even no - attention to the latter... and vice versa. Thus, immediately, we have definitions of the word 'consciousness' which seem to be about different things. And perhaps that's because they are about different things!
We can say, instead, that definitions of consciousness should include both what consciousness does and how conscious seems. Except, of course, that we'll then be in danger of supplying definitions which are far too long or detailed. In addition, some believers in the idea that consciousness is what consciousness does reject the phenomenal (or, say, qualia) outright – or see it as having little importance (at least to science). Similarly, those people who stress experience, subjectivity, qualia, “what it is like”, etc. may underplay what consciousness does; though, in this case at least, they're highly unlikely to ignore (or reject) the functional, neuroscientific, etc. aspects of consciousness.
*) To follow: 'On Definitions of 'Consciousness': Examples (2)'.