Just over a week ago, I posted a brief modal argument for mind-body dualism. The argument, as Alvin Plantinga presented it in the YouTube clip I posted, stands or falls with the crucial premise that “what is imaginable is logically possible.”
This prompted reader Jeremy to post a long, and thoughtful, comment on the subject of conceivability and logical possibility. He points to three potential counterexamples:
1. I can conceive that 2+2 might equal 4, not 5 [sic]. How? I imagine pushing two sets of 2 pennies together, and when counting them up, counting 5. I make sure not to imagine an extra penny appearing out of nowhere.
I think a distinction needs to be drawn here between phenomenology and counting. In the thought experiment, I will continually see four pennies, though I may count five. There is no doubt that the counting is dependent on what I have imagined, but it is not a part of it. (Consider: When presented with a large number of objects, we are often at a loss to estimate their number, even when all of them are easily within our sight.)
I cannot, then, conceive of there being two and two pennies brought together, which are five; though I can certainly imagine bringing the two and two pennies together, and counting (as I move between them) “1, 2, 3, 4, 5.”
But might we, in a similar spirit, say that what I am able to imagine is not the non-existence of my body as such, but its absence from my phenomenal experience? I can imagine floating from room to room, without ever witnessing my hands, feet, legs, or any other part of my body. I can imagine “appearing” before other people, who nonetheless seem not to notice me. But even if, in my phenomenal experience, I have no body, might I not still have one in the hypothetical physical world this thought experiment is situated in? I could, for example, be hallucinating in my hospital bed; or I might be a brain in a vat. The real trouble is that when we reflect deeply on what it means for the body to exist and what it means for subjective experience to exist, there still seems to be no contradiction in the one existing without the other.
2. . . . . I can conceive of having been born as Napoleon. Not that my consciousness dwells in Napoleon’s head, but that I were the experiencer of Napoleon’s experiences. How? I imagine thinking Napoleon’s thoughts, looking out of his eyes, having his temperament, etc.
I don’t see the problem. It seems logically coherent that any of us might have been born Napoleon, if by that one means “I experienced…” (insert a complete description of Napoleon’s mental life).
3. I can conceive that the Riemann hypothesis is false (or true, if you believe that it’s false). It’s easy to imagine how: I imagine a professor announcing a proof, it being validated.
Here, we do not really imagine “it being validated” per se. What we imagine is the mathematician’s fellow professors announcing excitedly that his proof is sound. That certainly is logically possible, even if the existence of an authentic proof is logically impossible.
Jeremy closes with the following note:
At best, I think conceivability can be used in the absence of other lines of reasoning to keep us from ruling out logical impossibility, but as soon as we have other lines of evidence, that should be weighted much more heavily.
This is actually not far from my position. As I wrote in my original post, “[t]he question of when we are justified in inferring logical possibility from conceivability is a difficult one; though we certainly do infer the one from the other on a regular basis.”
That is why I preferred to rephrase the premise in terms of a presumption. Perfect conceivability does, I think, guarantee logical possibility; but we humans rarely grasp concepts in their fullness. (To choose a feeble example, what does it mean to “think of” infinite space? There is a sense in which we have the concept of infinity, and a sense in which we do not.) The trouble, for the materialist, is that the “other lines of evidence” he is fond of citing are not really evidence of logical impossibility, but of nomic impossibility. And that is another matter entirely. It is logically possible that the gravitational constant might have been greater than it is, even if it is nomically impossible. Etc., etc.