This aphorism has been paraphrased and attributed to several people. Paul Valery, the French poet and essayist, and the famous artist and inventor Leonardo Da Vinci are the two main ones. Some have even attributed the saying to Pablo Picasso. In any case, whomever said it, and all three of them could have easily said it, it's a concept worth thinking about.
Google defines an aphorism as "a concise statement of a scientific principle, typically by an ancient classical author." Nothing in creation is ever truly complete, and this is something that Valery definitely wrote about.
This is a quote from one of Paul Valery's dramas, Le Cimetière Marin:
“In the eyes of those lovers of perfection, a work is never finished—a word that for them has no sense—but abandoned; and this abandonment, whether to the flames or to the public (and which is the result of weariness or an obligation to deliver) is a kind of an accident to them, like the breaking off of a reflection, which fatigue, irritation, or something similar has made worthless.”
There comes a point where a work of art must be abandoned in order to move onto another thing. This usually happens when improvement upon that work does not seem possible at the time. No work is ever truly perfect. But, there is always a sort of forced completion to every piece of art, whether it's painting, sculpting, writing, or otherwise.
Something to keep in mind for all of the writers and artists out there: you can always come back and improve something later. Some people obsess over perfection, but sometimes the flaws are part of what makes art what it is. The best part is once the artist or writer abandons a piece of work, sometimes others will continue to enjoy it for many years to come.
Not every piece of art or writing will turn out as you might expect. But, when you abandon your perceived failures, make sure to keep in mind those things that seem failures now could one day be improved upon to become wild successes.