Full Idea
When you have proved something you know not only that it is true, but why it must be true.
Gist of Idea
Proof shows that it is true, but also why it must be true
Source
John Mayberry (What Required for Foundation for Maths? [1994], p.405-2)
Book Reference
'Philosophy of Mathematics: anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.405
A Reaction
Note the word 'must'. Presumably both the grounding and the necessitation of the truth are revealed.