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I was recently listening to this great discussion on consciousness between Philip Goff and Richard Brown, and when they got to a discussion of conceivability arguments (e.g. zombie conceivability), Goff made the point that it’s a terrible name, because it sounds like it just has to do with what you can imagine. In fact, the point is about what is rationally coherent.

Within a day or so of listening to this discussion, I saw the following tweet from a philosophy PhD student:

This seems like a good example of Goff’s point about conceivability being a misleading name. Again, it’s not about what you can imagine, but about what is rationally coherent. And indeed, rational coherence does entail metaphysical possibility.

It’s useful to differentiate between two types of possibility:

• nomological possibility: if something is nomologically possible, that means it is possible in our world, with our laws of nature.
• metaphysical possibility: if something is metaphysically possible, that means it is possible in some possible world, e.g. perhaps a world with different laws of nature than our own.

For example, it doesn’t seem possible for us to travel faster than the speed of light. It’s a nomological impossibility. But it is metaphysically possible, e.g. it could be true in a different possible world. At least, it is not a logical contradiction.

In contrast, a square circle is a metaphysical impossibility. It could not exist in any possible world.

So where do conceivability and imaginability come into it? The idea is that you can’t truly conceive of a metaphysical impossibility, because it’s not rationally coherent. You might think you can conceive of a square circle, but you can’t. You could quickly switch back and forth between thinking of squares and thinking of circles, or meditate on the phrase “square circle,” but you can’t truly conceive of such a thing existing. But you can conceive of travelling faster than light.

So what we truly care about is rational coherence, and since we can be mistaken about what we think we can conceive or imagine (as with the square circle above), perhaps it’s best just to avoid language about conceivability. Perhaps “rational coherence” arguments?