True love, like a ghost, is much talked of but seldom seen.

[Il est du véritable amour comme de l’apparition des esprits tout le monde en parle, mais peu de gens en ont vu.]

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

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶76 (1665-1678) [tr. Stevens (1939)]
    (Source)

Present in the 1st (1665) edition; in that edition, the first phrase read more globally as "Il est de l’amour comme de l’apparition ..."

(Source (French)). Alternate translations:

It is with True Love, as with Ghosts and Apparitions, a thing that every body talks of, and scarce any body hath seen.
[tr. Stanhope (1694), ¶77]

It is with true love as with apparitions. Every one talks of it, but few have ever seen it.
[ed. Gowens (1851), ¶80]

There is real love just as there are real ghosts; every person speaks of it, few persons have seen it.
[tr. Bund/Friswell (1871)]

True love is like a ghost; everyone talks of it, few have seen it.
[tr. Heard (1917)]

True love is like psychic experience. Everybody tells ghost stories but few of us have ever seen a ghost.
[tr. FitzGibbon (1957)]

True love is like seeing ghosts; we all talk about it, but few of us have ever seen one.
[tr. Kronenberger (1959)]

True love is like ghostly apparitions: everybody talks about them but few have ever seen one.
[tr. Tancock (1959)]

It is with true love as it is with ghosts: everybody talks about it, but few have seen it.
[tr. Whichello (2016)]