Talking of a beautiful girl, a beautiful landscape, a beautiful picture, I certainly have very different things in mind. What is common to all of them -- "beauty" -- is neither a mysterious entity, nor a mysterious word. On the contrary, nothing is perhaps more directly and clearly experienced than the appearance of "beauty" in various beautiful objects. The boy friend and the philosopher, the artist and the mortician, may "define" it in very different ways, but they all define the same specific state and condition -- some quality or qualities which make the beautiful contrast with other objects. In this vagueness and directness, beauty is experienced in the beautiful -- that is, it is seen, heard, smelled, touched, felt, comprehended. It is experienced almost as a shock, perhaps due to the contrast-character of beauty, which breaks the circle of everyday experience and opens (for a short moment) another reality (of which fright may be an integral element).[Herbert Marcuse, One-Dimensional Man, 2nd edition. Beacon Press (Boston: 1991) pp. 210-211.]
This description is of precisely that metaphysical character which positivistic analysis wishes to eliminate by translation, but the translation eliminates that which was to be defined. There are many more or less satisfactory "technical" definitions of beauty in aesthetics, but there seems to be only one which preserves the experiential content of beauty and which is therefore the least exact definition -- beauty as a "promesse de bonheur." It captures the reference to a condition of men and things, and to a relation between men and things which occur momentarily while vanishing, which appear in as many different forms as there are individuals and which, in vanishing, manifest what can be.
The claim that beauty is a "promesse de bonheur" (promise of happiness) comes from Stendhal's Rome, Naples, and Florence, but was made more widely known first by Baudelaire in The Painter and Modern Life and then by Nietzsche in On the Genealogy of Morality.