Your Favorite Roman Latin Quotes Maxims Mottos Sayings ?

[Salvianus]

Perhaps one of you might know if there is a latin antecendant to the Mark Twain quote: "They did not know it was impossible, so they did it!"

For some reason I think there is, but can't source it.

I thought I had it somewhere:

possunt quia posse videntur
Often translated 'They can because they think they can.', but more literally: They are able because they are seen to be able.' (seem)
It's the Aeneid, Book V, line 231, describing one of the crews in the boat race. If I read it right they come second due to divine intervention!