Between campaigning for a society based on rationality rather than religion, Diderot defined the character of 'the genius'. Across several of his reflections on theatre (including The Paradox of the Actor, a still fascinating reflection on the art of performance and Entretiens sur le fil Naturel, a justification of his own script about blood and bastards), he described the qualities necessary for the creation of the finest art. Roughly, it's a combination of extreme sensitivity and the ability to reflect on intense experience, and it became the only part of his dramaturgical investigations that interested the Romantics.
The genius, back in classical times, had been a kind of local deity - a spirit of place. Diderot had no time for the mystical, and preferred the genius as a great artist, capable of making those timeless, universal pieces that never go out of fashion. It's possible that he had Shakespeare in mind, since the anarchic structure of the Bard's tragedies and comedies mocked the rigorous, austere and rule-bound notions of French neoclassicism, a style Diderot associated with the ancien regime and its tyrannical religiosity. In place of the sacred text, he postulated a sacred artist. He didn't actually say that they had to be white and male, but the genius is a good strategy to affirm the value of canonical authors.
The Romantics reacted against the Enlightenment's fascination with reason, chasing after a sublime dynamism that manifested, for example, in the German sturm und drang plays or the wild associations of William Blake's poetry. The genius, however, suited their more emotional inclinations, and the nineteenth century saw the exaltation of geniuses across the arts, from Beethoven through to Tolstoy. Within theatre, Shakespeare continued to be celebrated as a paradigm of the playwright (even though his scripts were often presented in heavily edited formats and, in London, were the basis for extravagant spectacles rather than earnest attempts to get back to the spirit of the source).
Romanticism exhausted itself relatively quickly - unsurprisingly, since its level of excitement suggests a nursery fueled by sugar-based snacks - and naturalism turned up, with echoes of Diderot's Enlightenment ideals for dramaturgy. Respect for formal considerations - a feature of avant-garde music as well as modernist literature - resurfaced and the passionate dynamism of Romanticism became a choice rather than a movement.
But the genius continues. Contemporary theatre and pop music,
emphasise the personality of the creator: the habit of playwrights to name their work, or perform them themselves, rejects the collaborative nature of theatre to suggest an auteur and the praise for the playwright itself occludes the importance of the director, the actors, the set designer (although these too have their star systems).
The notion of genius is a right pain in the hoop. Discuss...