It belongs by definition to the idea of a triangle that its angles total 180 degrees; it belongs by definition to the idea of a mountain that it is adjoined by a valley; likewise, it belongs by definition to the idea of God (as a perfect being) that He possesses all perfect properties (including the property of ‘existence’).
Therefore it cannot be said without contradiction that the three angles of a triangle do not total 180 degrees; nor can it be said without contradiction that a mountain is not adjoined by a valley; likewise, it cannot be said without contradiction that God does not exist.
It follows that these three propositions about triangles, mountains and God are all necessarily true: to the subjects of these propositions (triangles, mountains and God) the predicates (‘having angles that total 180 degrees’, ‘being adjoined by a valley’, and ‘existence’, respectively) necessarily belong, because these predicates form part of the definitions of the respective subjects.
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