There is no logically possible world in which there exists both an unstoppable force and an immovable object. I lapsed earlier this week in privately thinking that perhaps such a world could possibly exist because I could imagine a world where some force, which had the property of being unstoppable, could exist along with an object having the property of being immovable, just in case that world was such that the force and the object never met (or, more strongly, could never meet – say because the force supervened on the immovable object). However, obviously that is mistaken because to say that a force has never stopped, is not now stopped, and will never be stopped, is not to say that it is unstoppable. To be unstoppable, or to have the property of being unstoppable, is to have a counter-factual property. The only way to make sense of the property ‘unstoppable’ is to say that in no counter-factual situation is it the case that the F with this property would stop. The same could be said of being immovable. However, that means that both these properties are counterfactual properties. (Perhaps Omnipotence is also a counter-factual property in this sense).
This leads, however, to another interesting conclusion, namely that there is no logically possible world with either one of these two properties (or, more formally, where any being F has either one of these properties). Why? Well, because so long as it is not incoherent to suggest that some object is immovable, that will provide a counter-factual counter-example to some being’s being unstoppable (presumably a force – let’s be flexible with the term ‘being’ for the time being). The same can be advanced against an immovable object. So, so long as neither concepts are known to be incoherent by some reason other than the mutual exclusivity between them, it is the case that there is no logically possible world with either an unstoppable force, or an immovable object. It is also not possible for any being F to have both these properties, since we can postulate a counterfactual being F’ with both these properties which would act as a counterfactual defeater for F, such that if F’ did exist, F could not exist.