Full Idea
In every true affirmative proposition, necessary or contingent, universal or particular, the concept of the predicate is in a sense included in that of the subject; the predicate is present in the subject; or else I do not know what truth is.
Gist of Idea
A truth is just a proposition in which the predicate is contained within the subject
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.07.4/14)
Book Reference
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.63
A Reaction
Why did he qualify this with "in a sense"? This is referred to as the 'concept containment theory of truth'. This is an odd view of the subject. If the truth is 'Peter fell down stairs', we don't usually think the concept of Peter contains such things.
Related Idea
Idea 12910 The predicate is in the subject of a true proposition [Leibniz]