It often happens that things come into the mind in a more finished form than could have been achieved after much study.
 
[Il arrive souvent que des choses se présentent plus achevées à notre esprit qu’il ne les pourrait faire avec beaucoup d’art.]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶101 (1665-1678) [tr. Tancock (1959)]
    (Source)

Present in the 1st (1665) edition, where variants included the phrase:

There are pretty things that the mind does not seek, and that it finds all completed in itself; it seems that they are hidden there, like gold and diamonds in the bosom of the earth.
 
[Il y a des jolies choses que l’esprit ne cherche point, et qu’il trouve toutes achevées en lui-même; il semble qu’elles y soient cachées, comme l’or et les diamants dans le sein de la terre.]

(Source (French)). Alternate translations:

There are certain ingenious things which the mind seeks not after, but finds brought to their full perfection in it self; so that it should seem they lay hid there, as Gold and Diamonds do in the bosom of the earth.
[tr. Davies (1669), ¶190]

It often happens, that some things offer themselves to our Wit, and are finer description in the very first thought, than it is possible for a man to make them by the Additions of Art and Study.
[tr. Stanhope (1694), ¶102]

It often happens, that some Things offer themselves finer in the very first Thought, than it were possible for a Man to have made them by Art and Study.
[pub. Donaldson (1783)]

It often happens, that things present themselves to the mind; more finished, than we, with much labour, can make them.
[ed. Gowens (1851), ¶391]

It often happens; that things present themselves to our minds in a more complete state than we could by much art make them arrive at.
[tr. Bund/Friswell (1871), ¶104]

Ideas often flash across our minds more complete than we could make them after much labour.
[tr. Heard (1917), ¶101]

It often happens that ideas reach our mind in a state of perfection exceeding that which our intellect, with all the resources of art, could fashion.
[tr. FitzGibbon (1957), ¶101]

Things often spring spontaneously to mind in a more finished form than could be achieved with great labor and thought.
[tr. Kronenberger (1959), ¶101]