No, by ‘conceptually possible’ I mean ideally conceptually possible, i.e. a priori coherent, or free of internal contradiction. (Feel free to substitute ‘logical possibility’ if you are more familiar with that term.)
Before we discovered that water was H2O, our concept of water did not include that it was H2O. Since our concept did not include that, then surely it would not have been incoherent, at the time, to say that water is not H2O (imagine that this occurs during the period after the discovery of H and O and before the discovery of the composition of water—imagine that there was such a period), since there was nothing in our concept of water at the time that logically contradicted that statement. However, today it is incoherent to say that water is not H2O, because our concept of water includes that it is H2O—water is regularly defined as H2O.
Let us think about the period when, because the concept did not include that it was H2O, it was free of internal contradiction to say that water is not H2O, and therefore logically possible, ideally conceptually possible, and a priori coherent, to say that water is not H2O. Given their concept of water and perhaps even given everything they knew at the time about water, it was logically possible—that is, not in logical contradiction to any fact or concept they possessed at the time—that water is not H2O. Looking back, I find myself reluctant to draw any deep lessons from this about a possible dual nature of water in which, say, H2O takes the role of the material substance and water takes the role of the epiphenomenon. I find myself moved not at all to contemplate the possibility of zombie H2O—H2O which is not water. The only lesson I find myself wanting to draw from this is that they did not know then what we know now.
Now we turn to the present moment, when some claim that a zombie world is logically possible. They may be right to claim that their own concept of consciousness does not logically contradict the denial of consciousness to a physical creature. It is not at all obvious that I should draw any deep lesson from this, any more than in the case of water.
Before we discovered that water was H2O, our concept of water did not include that it was H2O. Since our concept did not include that, then surely it would not have been incoherent, at the time, to say that water is not H2O (imagine that this occurs during the period after the discovery of H and O and before the discovery of the composition of water—imagine that there was such a period), since there was nothing in our concept of water at the time that logically contradicted that statement. However, today it is incoherent to say that water is not H2O, because our concept of water includes that it is H2O—water is regularly defined as H2O.
Let us think about the period when, because the concept did not include that it was H2O, it was free of internal contradiction to say that water is not H2O, and therefore logically possible, ideally conceptually possible, and a priori coherent, to say that water is not H2O. Given their concept of water and perhaps even given everything they knew at the time about water, it was logically possible—that is, not in logical contradiction to any fact or concept they possessed at the time—that water is not H2O. Looking back, I find myself reluctant to draw any deep lessons from this about a possible dual nature of water in which, say, H2O takes the role of the material substance and water takes the role of the epiphenomenon. I find myself moved not at all to contemplate the possibility of zombie H2O—H2O which is not water. The only lesson I find myself wanting to draw from this is that they did not know then what we know now.
Now we turn to the present moment, when some claim that a zombie world is logically possible. They may be right to claim that their own concept of consciousness does not logically contradict the denial of consciousness to a physical creature. It is not at all obvious that I should draw any deep lesson from this, any more than in the case of water.