Who knows truly? Who here will declare whence it arose, whence this creation? The gods are subsequent to the creation of this. Who, then, knows whence it has come into being?
Whence this creation has come into being; whether it was made or not; he in the highest heaven is its surveyor. Surely he knows, or perhaps he knows not.
Not sure if this will qualify as a rationalist quote, but these are the last few lines from the Creation Hymn in the Rig Veda, the oldest of the Hindu sacred texts & estimated to be composed around 1100 BC. I like the note of uncertainty, rather rare among religious texts.
In its original, atheist Carvaka writings contained much verse (as Indian philosophy/theology usually does); see http://www.humanistictexts.org/Carvaka.htm In translation, they almost sound like senryū:
If a beast slain as an offering to the dead
will itself go to heaven,
why does the sacrificer not straightway offer his father?
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.