Relation is defined by St. Thomas as ordo unius ad aliud, in which three elements – the subject (the thing that is referred), object (the thing to which the subject is referred) and finally the cause of this referring (ground or fundament) – can be distinguished. Though relations are real, they are dependent in the order of existence upon their elements and, in a similar way as accidental properties, are forms of being secondarily added to a substance. Being of a relation is added to that of a substance and connected with it. Relation existing by virtue of its subject is the most imperfect being (imperfectissimum esse). It is the weakest form of beingness (esse) because it requires not only previous existence of a substance but also existen...