It has been argued that if the mind really is identical with some physical process (materialism is correct) then it must be a necessary fact that certain physical processes are conscious. Let us accept this claim for the moment. Some would then argue that, because it is conceivable that these physical processes are not conscious, it is not a necessary fact, and thus that materialism is false.
Let’s take a closer look at this argument. We all agree that it is a necessary fact that all unmarried men are bachelors, by the definition of the words. However, it is also possible that there might be some people who didn’t fully understand what being a bachelor entailed, and thus think that it is possible for some unmarried men not to be bachelors. Similarly, I think it possible that we simply don’t understand consciousness fully, and thus may very well be mistaken in claiming that it is possible for certain physical processes to lack consciousness.
Normally we could settle this by detailing the truth conditions for both terms (in all possible worlds), and if they were the same we would conclude that it really is the case that one necessarily is the other. For example, we could consider all possible men, determine if each is a bachelor and unmarried, and, when we determine that there are no unmarried men who aren’t bachelors (and no bachelors who aren’t unmarried), we could conclude that unmarried men and bachelors really are necessarily the same.
Unfortunately, this method can’t settle our questions about consciousness. For one we use our pre-analytic judgments about consciousness to determine which physical processes should be considered conscious (at least in the beginning of our investigations), so when considering which physical processes in this world are conscious our judgments are guaranteed to coincide with our concept of consciousness, simply because of the way in which we investigate the physical basis of consciousness. And we can’t rationally consider possible worlds (or at least some possible world), for example “a world exactly physically identical to ours, except where no one is conscious, only an automation”, because they beg the question (by supposing that such worlds really are possible, when all we know is that they are conceivable). If materialism is correct such possible worlds are really logically impossible worlds, just as a world where “there are some unmarried men who are not bachelors”. On the other hand, if materialism is incorrect such worlds may be possible; to know which we must settle the original question: “are certain physical processes necessarily conscious?” and thus appeals to possible worlds are out of the question, since circular reasoning is generally frowned upon.
Perhaps then we should re-examine the original question. Consider a similar question: “is the cause of iron bars falling to the ground necessarily gravity?” The answer to this is no, there are possible worlds without gravity, where iron bars fall because of giant magnets embedded in the ground. But this doesn’t change the fact that the cause of the iron bar falling really is gravity, and that we are perfectly justified in identifying the cause of the iron bar falling with gravity. What I am proposing then is that since we are interested in consciousness in this world the possibility of consciousness not being identified with certain physical processes in strange other worlds shouldn’t concern us. There is still a sense, however, in which the cause of the falling iron bar is necessarily gravity, at least in this world, and it is this sense of necessity that must hold in the relationship between consciousness and certain physical process if materialism is to be a correct description of the world. I will call this kind of necessity necessity*.
Necessity* is best described as a statement holding in all physically possible worlds, where physically possible worlds are those in which the natural laws are the same but the contents of the universe may be different (although the definition of necessity found in conditional logics would work too). Now if it really a necessary* fact that certain physical processes are conscious then they will be conscious in all physically possible worlds. For the moment let us assume that we do have a complete and accurate theory about which physical systems are conscious. Of course we don’t have such a theory now, but this shouldn’t influence our conclusions about necessity*, since it was still a necessary* fact that gravity was the cause of iron bars falling to earth, even before a complete theory about gravity was developed. Given such a theory, if it is to be the case that these physical systems are not necessarily* conscious then it must be the case that at least some of them can lack the property of being conscious in some physically possible world. This means that some system that is conscious in our world (since it is about such systems that the theory was developed to describe) is physically identical to some system that is not conscious in some other world. But since they are physically identical, and the worlds obey the same natural laws, it will be impossible to distinguish between them, for example they will behave identically. And if it is impossible to distinguish between some conscious and non-conscious systems we are effectively saying that we really have no idea when something is conscious, since we have just admitted that it is entirely possible for other people to be non-conscious, at least as far as we know. Such a position is at best silly, and at worst equivalent to admitting that the word consciousness has no meaning (since you can’t specify the truth conditions for it). So if we can create a theory that determines which physical systems are conscious, correctly to the best of our knowledge, then it is a necessary* fact that certain physical processes are conscious. On the other hand if such a theory can’t be developed then it isn’t a necessary* fact. At the moment things seem to be favoring a complete theory of consciousness, but I guess we’ll have to wait and see to be sure.