This post is divided into two parts. In part 1 I will discuss how various philosophical positions on the mind and consciousness would answer this seemingly ridiculous question (and perhaps their answers to it should cause one to doubt their validity). Part 2 is more serious, and focuses on showing why a description, for example a functional description, cannot be given to an arbitrary system, thus showing that rocks cannot be conscious under certain views of what consciousness is.
Dualism is the position that the mental is a completely different thing from the physical. In some theories the mental is considered to be a separate substance, subject to its own laws, while in other theories the mental is separate from the physical because the mental properties are completely independent of the physical properties. In either case the physical and the mental are only tenuously connected at best. Because of this most dualists would be forced to concede that it would be possible for rocks to be conscious. After all, the mental substance or mental properties are not dependant on physical properties, so there is no reason to say that the rock could not be conscious. It’s hard to say if it is even unlikely for the rock to be conscious, because there is no test under dualism that could be performed to determine what is and what is not conscious. (If you believe that such a test might exist you are no longer a dualist.)
Idealism is the view that everything is part of the mind. Idealists hold that the world does not exist independently of us because it is defined completely by our perceptions of it. Although idealism seems preposterous there are a handful of intelligent people who defend it, and thus it would be rash to dismiss it out of hand. Unfortunately under idealism it is nearly impossible to have certain knowledge about anything, since none of our perceptions have access to the underlying reality. Even under systems of idealism which hold that other minds exist independently of ourselves it is hard to identify what aspects of the world represent interaction with those minds. Thus under idealism it is possible that the rock may be one of the other minds, but you would never be able to find out if it was, so it is somewhat of an empty question.
Behaviorism is a theory in which consciousness is identified with certain types of behavior, especially linguistic behavior about beliefs and thoughts. Under behaviorism we could say that rocks are not conscious, because they do not display any kind of behavior.
Identity theory is a position which holds that consciousness is identical with certain physical properties of the brain. Under identity theory a certain pattern of neural activity is the same thing as a certain thought. There are also less strict versions of identity theory in which certain classes of neural activity can be identified with a thought. At first you might think that this rules out the possibility of conscious rocks, since there is nothing in a rock that could count as the correct type of neural activity. However identity theory doesn’t rule out other kinds of consciousness, such as alien consciousness, that are identified with completely different physical features of the alien’s “brain”. Thus it is possible under identity theory that there exists a kind of rock-consciousness that can be identified with various physical properties of rocks.
Biological naturalism is the view that consciousness is a product both of our biology and the process of evolution. Biological naturalism can be rather difficult to understand so I will skip over the details. It is enough to say that rocks cannot be conscious under biological naturalism since the rocks present now did not come about as a result of an evolutionary process from earlier rocks, and thus it is not possible for them to be conscious.
Epiphenomenalism is the view that the mind is somehow caused by the operation of the brain. Unfortunately epiphenomenalism doesn’t say exactly how the brain causes the mind to exist, and thus it is possible that rocks somehow also cause minds to exist.
Functionalism is the view that consciousness can be identified with some operations of the brain. However functionalism holds that it is the nature of the operations (i.e. how they processes information) that is to be identified with consciousness (in contrast to identity theory which identified consciousness with some specific properties of the brain). In theory anything that performed the same operations would be conscious, weather it be human brain or computer. Since a rocks does not process information we can conclude that there is no functional description associated with them, and thus that rocks are not conscious.
There is one problem with my argument that functionalism shows rocks to lack consciousness, and that is the assumption that there is no computational description that can be given to a rock. In fact any theory that relies on descriptions needs have criterion determining when it is justified to apply a given description and when it is not. The problem is that without such a criterion it may seem that anything can be given any description. For example consider the a small section of the brain shown at several time intervals, where . is inert matter and – is an electrical charge.
... ... ... -.. .-. ..- ... ... ...
We might describe this situation as the movement of a signal from one part of the brain to the other. On the other hand consider a rock, where each . is an atom.
... ... ... ... ... ... ... ... ...
Since it doesn’t look like there is any activity me might think that there is no signal propagation. But consider the case where we suddenly decided to label one atom in each time interval as special.
... ... ... #.. .#. ..# ... ... ...
Now it seems that we do have a description of the rock that indicates signal propagation. We might extend this kind of description to the whole rock, at each instant distinguishing some atoms from the rest, so that when viewed as a whole we could give the rock a computational description that indicates it has consciousness.
To make the claim that the computational description of our brain is valid and the one of the rock is not we have to make some distinctions. First we can separate out the computational description from our physical descriptions, weather it be the charged/unchanged description of the brain or the special/non-special atoms description of the rock. We can say that given either of these descriptions the computational description is warranted, meaning that we now need to show why the charged/unchanged description is different from the special/non-special description. The key difference between these two is where the information that makes up each description comes from. In the charged/uncharged description the information comes from a rule and the physical properties of the system (the rule having to do with the ratio of electrons to protons). In contrast the description we are trying to apply to the rocks gets its information completely from the rules; the rules tell us what kind of description we need to justify a computational description on top of it, and so we label the atoms appropriately. We can then reject the second kind of description because it is independent of the physical basis, which is contrary to our intuition that a valid description must have a basis on what it is describing.
There is also a slightly more interesting, and difficult, case to ponder over, which may cast some doubt as to our criterion for what is a valid description. Consider a computational description of water, where the movement and interaction of water molecules is the basis for such a description (meeting our criteria). Usually the computational description is meaningless, but occasionally the bucket may be shaken in just the right way that for a few moments the computational description is meaningful, and possibly even indicative of consciousness. Is this really plausible? I think it might be, even though it is so improbable that it would never happen, but I will leave a more detailed analysis of this for another time.