Full Idea
If being is what makes propositions true, then anything we can express in an affirmative proposition, however unreal, is said to be; so lacks and absences are, since we say that absences are opposed to presences, and blindness exists in an eye.
Gist of Idea
If affirmative propositions express being, we affirm about what is absent
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.92)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.92
A Reaction
See Idea 11194 for the alternative Aristotelian approach to being, according to categories. Do absences and lacks have real essences, or causal properties? The absence of the sentry may cause the loss of the city.
Related Idea
Idea 11194 Being is either what falls in the categories, or what makes propositions true [Aristotle, by Aquinas]