Claudia Moscovici 
Diderot first broached the question of what is beauty in an article of the Encyclopédie that was published on January 21, 1752 entitled, appropriately enough, “Beauty.” He then edited and developed his arguments further in the Traité du beau, which was published twenty years later. This philosophical treatise considers the arguments about beauty presented by the British empiricists Shaftesbury and Hutcheson. What appeals to a materialist such as Diderot about their writings is the understanding of beauty as the product of an infinite number of repeated experiences. Like Diderot, Shaftesbury proposes a refined empiricism that applies the insights of inductive models of knowledge to age-old aesthetic problems, the most important of which is elucidating the nature of beauty. In broaching this subject, Shaftesbury himself relied upon Plotinus’ neoplatonic philosophy to reconcile empiricism with idealism, or, more specifically, the variety of sensory impressions with an everlasting, unified and universal idea of beauty. The beauty of art and of nature, Plotinus had claimed, reflects a higher, divine harmony. Like his precursor and inspiration Plato, the Renaissance philosopher climbs upon an idealist ladder that leads from physical sensation to pure form; from the particular to the universal; from the individual to the world soul, or what Plato had called the Good.
Shaftesbury is not an idealist like Plotinus. Nevertheless, he too sees the beauty of nature and of art as a reflection of the higher harmony and meaning of the cosmos. In appreciating beauty, Shaftesbury further suggests, we’re not simply passive beings that absorb sensory impressions, but rather creative individuals who exercise judgment and taste. The appreciation of beauty—at least according to the philosophical tradition that leads from Plato to Diderot—is therefore sensory but not merely physical. It engages our faculties and reveals the underlying harmony of the world.
Even assuming that this conception of beauty were true, an immediate question presents itself: how do we gain access to this higher realm; how do we discern the beautiful? Finding himself in agreement with Shaftesbury, Diderot maintains that the appreciation of beauty is instilled, first and foremost, by repeated observation. Thus like the empiricists, Diderot founds aesthetics—which means, as mentioned, “of sense experience”—appropriately enough upon perception. Nonetheless, it can be objected that observation in itself doesn’t give us any particular direction: seeing things repeatedly doesn’t even indicate that we necessarily like them, much less imply good taste. To address this objection, Diderot offers his own definition of beauty, one that combines Neoclassical standards with empiricist assumptions. Beauty, the philosopher states, is a harmony between the parts and the whole; or, phrased more in line with his materialist theory of knowledge, it’s the perception of the relation of unity between the parts and the whole. This understanding of beauty is broad enough to apply to a vast array of things: natural objects, art, feelings, human beings.
Yet, one could further object, if we all appreciate order and harmony in similar ways, what is one to make of the obvious variations in standards of beauty? Diderot is preoccupied with this problem. He raises the fundamental question, is beauty “something absolute or relative?” (Traité du beau, 81) Or, otherwise put, is there an unchangeable, eternal, essential beauty or is beauty like fashion, variable and dependent upon shifting tastes? (81) Ultimately, as Baudelaire would do several generations later, Diderot settles upon both. If beauty were eternal, there would be no way to explain changing criteria. If it were ephemeral, it would be a product of ignorance and, as he states, “throw the whole philosophical question into sheer scepticism” (81). A good way of explaining more tangibly the abstract claim that beauty is both particular and universal, Diderot implies, is by considering one’s judgments of concrete objects of beauty, such as paintings. Which is precisely what he does in the Salons.
https://rowman.com/ISBN/9780739116753/Romanticism-and-Postromanticism
