Full Idea
Statements about that which is stable, secure and manifest to the intellect are themselves stable and reliable (and it's important for statements about such things to be just as irrefutable and unassailable as statements can possibly be).
Gist of Idea
Some statements about what is obvious and stable are as irrefutable as possible
Source
Plato (Timaeus [c.362 BCE], 29b)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Waterfield,Robin [OUP 2008], p.17
A Reaction
This is a nice thoughtful account of what we mean by a necessary truth, without attributing to it an absolute character.