All your armchair Latin experts take a seat and let the master do his job!
In Classical Latin, sum
can mean both "I am" and "I exist." However, the Latin word existo
, (NOT EXSTO)depending on who uses it in Classical Latin, refers more to a sudden coming into being. It is related to the verb sto
which means, basically, to stand. So, there is a fundamental difference between the two Latin verbs. However, I cannot speak more surely about post-classical Latin. I would submit that by the time of Descartes there was a more subtle and fine line drawn between existence and being. Funny thing is that prior to Aquinas, the Latin language had no word for "being." The present participle form of sum didn't exist in the Classical vocabulary which is probably why the Ancient Romans were not so good philosophers (exception being Lucretius). To rectify this, they used the present infinitive of sum
which is esse
but "to be" and "being" are not synonymous. Thus, I think that by Descartes' time since there was a Latin word to describe "being" using the Aquinas particple, ens
, whence come words such as entity, there was more exactitude pertaining to being and existing, the former referring more to a spiritual/mental sense and the latter to a more physical reality. mho.