  The analysis which De quiditatibus provides is subtle. The term “quid” denotes the essential mode of the being of a thing, that is, the mode of being by which it exists as a being of a certain kind. It answers the question “What is it?” On the other hand, quiditas denotes the formality by means of which a thing is a quid. It is tempting to see a connection here between ens, entitas and quid, quiditas such that the second is the condition of the first. This may lead unwary reasoners, as Dietrich no doubt felt Thomas Aquinas and others were, into equating entitas and quiditas. But quiditas denotes the formal determination of a thing. This is something over and above the being of the thing. It is a form, the form of a thing. But it is not the same as the thing. Hence it cannot be equated with either the ens or the entitas of the thing. Of course quiddity presupposes ens and esse; without its existence there could be no determination of a thing.