Full Idea
That which is rational is better than that which is not rational; but there is nothing better than the cosmos; therefore, the cosmos is rational.
Gist of Idea
Rational is better than non-rational; the cosmos is supreme, so it is rational
Source
Zeno (Citium) (fragments/reports [c.294 BCE]), quoted by M. Tullius Cicero - On the Nature of the Gods ('De natura deorum') 2.21
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.62
A Reaction
This looks awfully like Anselm's ontological argument to me. The cosmos was the greatest thing that Zeno could conceive.