Ce que l'on conçoit bien s'énonce clairement / Et les mots pour le dire arrivent aisément
Rough translation: "What is well understood can be told clearly, and words to express it should come easily."
ETA: it is worth pondering the converse; just because something rolls off the tongue doesn't mean it's well understood. It could be that it's only well-rehearsed.
What the quote is aimed at is work of a supposedly high intellectual caliber, which just so happens to be couched in impenetrable jargon. Far more often, that is in fact evidence of muddled thought, not that the material is "beyond me".
It's obvious, but I must point out that giving the quote in the original French and providing a "rough translation" seems at odds with the message of the quote.
A monthly thread for posting rationality-related quotes you've seen recently (or had stored in your quotesfile for ages).
ETA: It would seem that rationality quotes are no longer desired. After several days this thread stands voted into the negatives. Wolud whoever chose to to downvote this below 0 would care to express their disapproval of the regular quotes tradition more explicitly? Or perhaps they may like to browse around for some alternative posts that they could downvote instead of this one? Or, since we're in the business of quotation, they could "come on if they think they're hard enough!"