Full Idea
A double contradictory is the contradictory of a contradictory, for example, 'It s not the case that it is not day'. It posits that it is day.
Gist of Idea
The contradictory of a contradictory is an affirmation
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.69
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.18
A Reaction
Seems like common sense to the stoics, but verifying the double negative may be a different procedure to verifying the affirmative. 'Are you happy?' 'Well ….I'm not unhappy'. 'Is it day yet?' 'Well, it's not night'.