Even though you are correct and logicians like Avicenna and Peter of Spain wrote in gory detail about (what we take to be) Medieval Modality[1], this does not m

Even though you are correct and logicians like Avicenna and Peter of Spain wrote in gory detail about (what we take to be) Medieval Modality[1], this does not make Medieval Logic even remotely as rigorous as its modern incarnation. The issue here is about having a sound, consistent, and complete logic. S5[2] (what Godel used) has those properties, whereas Anselm didn't even have the notion of soundness or completeness, let alone a logical system that would satisfy them. [1] http://plato.stanford.edu/entries/modality-medieval/ http://plato.stanford.edu/entries/modality-medieval/ [2] http://en.wikipedia.org/wiki/S5_%28modal_logic%29 http://en.wikipedia.org/wiki/S5_%28modal_logic%29