Tuesday, August 07, 2007

Actual Possibilities, by Jonny Blamey

When people try to explain to me what is meant by metaphysical possibility, and possible world semantics, they tell me that possible worlds are “ways the world could be”. I feel like I understand. But when I say what I understand by “the ways the world could be” I am usually accused of mixing up metaphysical modality with epistemic modality. My intuitive understanding of “the ways the world could be” is actually a misunderstanding, since intuitively (to me) the ways the world could be is an epistemic notion. What is more “the world” in “the ways the world could be” is the actual world. I have no interest, certainly no practical interest, in the ways some possible world could be. I am a fan of science fiction, my interest is excited because the scenarios presented could happen here, in this world, albeit in the future. I equally enjoy fiction set in historical times. If the book is well researched then the events portrayed could have happened, they are ways this world, my world, could have been. The probability, according to my evidence, that Rob Roy cut an Englishman in half, like in the film, is zero and I am quite aware of this. But he could have done. The probability that Rob Roy became King of England is likewise zero, but this could not have happened, this is not a way the world could have been. If someone were to tell me that Rob Roy could have become king of England, I should respond by asking how. I would then sit back and enjoy listening to what facts would need to be unpicked, what chance happenings would have had to have occurred for Rob Roy to capture the throne. This pathway, to be of any interest to me, would have to be picked and unpicked out of the fabric of this world, my world, the only world there is.

Enough preamble. Here are two arguments that I take to be both modal and valid, but that seem to be about about the actual world and notabout possible worlds at all but . Neither are they about epistemic modals given the wonderful principle laid out by Moritz Schulz in this blog: (MIGHT c(might x) = 1 if c(X) > 0.)


Premise: If John had set his alarm for eight o’clock he would have got here on time.
Premise: John could have set his alarm for eight o’clock.
­­­­­­­­­­­­­­­­­­­­­­­Conclusion: John could have got here on time.

The conclusion is modal. We are to assume that in fact John did not get here on time. It is not consistent with our evidence that John got here on time, so this is not epistemic possibility. The probability that John got here on time is 0. So according to MIGHT it is not true that John might have got here on time. However, there is surely a possible world where John got here on time whether he set his alarm clock or not. But the conclusion of this argument is not trivially true. The second premise is necessary for the conclusion to follow.


Premise: The only way that John could have got here on time would have been by car.
Premise: John does not have a car.
Conclusion: John could not have got here on time.

This time the conclusion is not counterfactual, since it is assumed that John did not get here on time. So the probability that John did not get here on time is 1. We again are not using epistemic modals here since according to MIGHT John must have got here on time and the conclusion is redundant. In this second argument, the second premise must be true in the actual world for the argument to go through. Are there possible worlds in which John has a car? In these worlds he could have got here on time. But we know that in this world he doesn’t have a car, so in this world he couldn’t have got here on time.

These modal arguments are important, especially in assessing culpability, but also in improving design and assessing scientific evidence. But they don’t seem to fit either metaphysical modality nor epistemic modality.