David Lewis held that possible worlds are worlds as concrete as our own (cf. Lewis (1986)). He also held, in his work on counterfactuals (cf. Lewis (1973)), that all counterfactuals with impossible antecedents - 'counterpossibles' - are vacuously true. These two views do not fit together well. Embracing Modal Realism leads to especially compelling counterexamples - counterexamples given modal realism, that is - to the thesis that counterpossibles are all true. These take the form of conditionals whose antecedents are not intuitively impossible, but which are impossible given modal realism.
These arise because, according to modal realism, reality as a whole – that is, the totality of the posited worlds – is necessarily the way it is. Lewis is very upfront about this. Witness:
So, for example:
'If there had been two fewer men in reality as a whole than there actually are, there would have been fewer women.'
There is no reason to think this is true. And yet Lewis's thesis about counterpossibles, together with modal realism, implies that it is vacuously true.
Perhaps worse:
'If there had been fewer men in reality as a whole than there actually are, there would have been just as many men in reality as a whole as there actually are.'
This seems positively false.
References
Lewis, David K. (1973). Counterfactuals. Blackwell Publishers.
Lewis, David K. (1986). On the Plurality of Worlds. Blackwell Publishers.
These arise because, according to modal realism, reality as a whole – that is, the totality of the posited worlds – is necessarily the way it is. Lewis is very upfront about this. Witness:
There is but one totality of worlds; it is not a world; it could not have been different. (Lewis 1986: 80.)
So, for example:
'If there had been two fewer men in reality as a whole than there actually are, there would have been fewer women.'
There is no reason to think this is true. And yet Lewis's thesis about counterpossibles, together with modal realism, implies that it is vacuously true.
Perhaps worse:
'If there had been fewer men in reality as a whole than there actually are, there would have been just as many men in reality as a whole as there actually are.'
This seems positively false.
References
Lewis, David K. (1973). Counterfactuals. Blackwell Publishers.
Lewis, David K. (1986). On the Plurality of Worlds. Blackwell Publishers.
