Entry 0198: Actus Essendi and Existence (IV)
The Notions of Existence and Actus Essendi
"Modern philosophers have failed to take seriously the difference between
(a) the kind of existence implied whenever we make a statement about a logical subject, e.g., about blindness, about a hole, about a colour or a number, or about a set, viz. that blindness exists, that the hole exists, that there is such a colour or number or set, and
(b) existence in the sense of the present actuality of something real enough to 'act.'
"In this sense they have ceased to talk about existence in the sense which most concerned Saint Thomas: 'esse in the sense of actus essendi.'
"Yet, this is the key to 'what is metaphysics' as the Pope [John Paul II] refers to it." 
↑ David Braine, “The Relationship Between Philosophy and Cultures,” “Reflections on the Encyclical Fides et Ratio,” no. 14, in L'Osservatore Romano English Weekly Edition, 4 August 1999, pp. 5-6.