Wisdom from the Early Stoics, Zeno of Citium 44

A proposition which indicates more or less is one that is formed by the word signifying "rather" and the word "than" in between the clauses, as, for example, "It is rather daytime than night." 

Opposite in character to the foregoing is a proposition which declares what is less the fact, as e.g. "It is less or not so much night as day." 

Further, among propositions there are some which in respect of truth and falsehood stand opposed to one another, of which the one is the negative of the other, as e.g. the propositions "It is day" and "It is not day." 

A hypothetical proposition is therefore true, if the contradictory of its conclusion is incompatible with its premise, e.g. "If it is day, it is light." 

This is true. For the statement "It is not light," contradicting the conclusion, is incompatible with the premise "It is day." 

On the other hand, a hypothetical proposition is false, if the contradictory of its conclusion does not conflict with the premise, e.g. "If it is day, Dion is walking." 

For the statement "Dion is not walking" does not conflict with the premise "It is day." 

—Diogenes Laërtius, 7.72-73