The what/which distinction is one I have always been a little hazy about in Latin, so do have patience if the answer to this is too obvious to be worth mentioning.
Doing some work on the more philosophical side of cosmology, there is a distinction to be made:
- The whatness of the laws of nature. For instance, "The strength of the force of gravity is X".
- The whichness of the laws of nature. For instance, "The strength of the force of gravity could have been X, or Y, or Z, and is in fact X".
I wonder if there are technical terms in philosophical Latin which would distinguish them.
The reason for making this distinction is that a "whatness" question has an answer that is complete in itself - perhaps "137". Asking "Why?" on top of that is not really an expected move in the game, and if the answer is "Because it just is like that", nobody minds very much. On the other hand, a "whichness" answer selects among a number of equally possible values and invites the supplementary question, "Given that they are all equally possible, why is the answer this rather than that?"
It is often helpful to try to translate terms back into Latin or even Greek, to get a handle on how people have been thinking about them before. The languages are often sharper, as well. But in this case, it appears to me that in Latin one deals with quid=what and quid=which by looking at the overall context of the sentence. And certainly quidditas ends up having a basically "whatness" meaning. That is all very well, but it means that one cannot package up the notion of whichness into a simple -itas.
I hope I am missing something very obvious here!
(And Greek being richer in philosophical terms than Latin, a Greek answer would be helpful as well.)
I should add that it is conceivable that "whatness of the laws" might well be better expressed as "what-kind-ness of the universe" - hence, qualitas – but that doesn't help with the whichness question and is, if anything, a step backwards.