Thalia was born among the nine Muses, daughters of Zeus and Mnemosyne. Hesiod names her third, between Euterpe and Melpomene. He does not yet label her the Muse of Comedy. Her name is connected with flourishing, and later Greco-Roman tradition gives her comedy and bucolic or pastoral poetry, represented by the comic mask, ivy and shepherd's staff.

Comedy grew up in her shade, and it grew out of the earth itself. The Greeks traced their laughter to the komos, the revel band of young men who sang and capered through the streets at the time of the grape harvest, carrying phalluses of leather and gourd and mocking whoever deserved it. The revel belonged to Dionysus, the god of wine and ecstasy, the twice-born deity whose festivals loosened every tongue in the city. Out of the komos songs the poets of the towns fashioned something new: a chorus of masked dancers and a handful of actors who impersonated the living, poking fun at generals, philosophers, and the gods themselves, all in verse of astonishing speed and filth. By the fifth century before the common era the city of Athens had built the revel into an art with prizes, staging comic contests at the Lenaea in winter and at the Great Dionysia in spring, and the greatest winner of all was Aristophanes, the towering poet of Old Comedy, whose plays lampooned the leaders of Athens in the middle of war and dared the city to laugh at its own folly. In a typical play of his, the action would halt and the chorus would turn to face the audience and lecture them directly on their politics, the famous parabasis, which no tragedy would ever permit. Comedy wore the opposite costume to tragedy's dignity: padded bellies and buttocks, grotesque masks with gaping grins, and in the earliest years a leather phallus flapping in the wind. Thalia's laughter was licensed misrule, the one night of the year when the low could mock the high, and the city called it healthy. In the generations after Aristophanes the wildness was tamed into the domestic comedies of Menander, full of mistaken identities and young lovers, which taught Rome its stagecraft and taught the modern world the sitcom. Every laugh that has ever been raised in a theatre with a story is still being raised, in a sense, in her precinct.

Pseudo-Apollodorus says Thalia bore the Corybantes to Apollo. The Corybantes, Curetes and other armed dancing groups are often brought close together in later mythographic tradition, but their identities and genealogies vary. Stories in which armed dancers conceal the cries of the infant Zeus belong to this wider Cretan complex and should not be presented as one universally fixed biography of Thalia's sons.

Her other province was the quiet one. Bucolic song takes its name from the boukolos, the herdsman, and it belonged to the hillsides at noon, when the flocks lay in the shade and the only work left was to sing. Thalia's pipe in this country was not the double flute of the city stage but the simple shepherd's pipe of reeds, and her crook was the badge of the men who used it. The art came fully alive centuries after her birth, when Theocritus, a poet of the island of Sicily in the third century before the common era, wrote his idylls, little pictures in verse of shepherds like Thyrsis and the beautiful, doomed Daphnis, who sang contests for prizes of milking pails and lamented their loves under the pine trees. Theocritus invented nothing out of nothing; he said the songs were already there, old as the hills, and he gave them his polish. Rome answered with the Eclogues of the poet Vergil, shepherds' songs polished to a sheen of gold, and through Rome the pastoral passed into every European language, where it flowered for two thousand years as the poetry of escape, the dream of a simpler life under Thalia's crook. From the comic mask to the shepherd's shade, her two provinces are secretly one: both are refuges where ordinary people, citizens and herdsmen, are allowed to stop being grand and simply be human.

The Greeks housed her with her sisters and set her emblems where the people could see them. At Delphi on Mount Parnassus, beside the Castalian spring where singers washed before they performed, the Muses shared an altar with Apollo. On Mount Helicon, in the Vale of the Muses, statues of the nine stood in a row, and the guides pointed out the springs where the songs began. Comic actors honoured her directly: masks were dedicated in shrines, and the professional guilds of actors and musicians, who travelled the Greek world under the name of the Artists of Dionysus, opened their meetings with the Muses and closed their performances with hymns. When Rome took the Muses to itself, it kept Thalia's name unchanged, and her mask became one of the most copied images of antiquity, carved on gems, woven into mosaics, and stamped on the lamps of the theatre districts. The Romans themselves were famously proud of their own rough tradition of improvised farce, and they grafted it onto her cult of laughter until the two could not be told apart. Through every century since, whenever a people has built a stage and allowed the mask to speak the truth the unmasked may not, Thalia has had a shrine there, whether her name is carved above the door or not.

Her name is still green. In 1852 the English astronomer John Russell Hind, mapping the small planets between Mars and Jupiter, discovered one and named it 23 Thalia, a year before he hung its sister 27 Euterpe in the same sky. Botanists gave her name to a genus of tall aquatic plants of the arrowroot family, reeds of the water meadows that flower where the land runs wet and rich, a Thalia that blooms quite literally. In 1843 the people of Hamburg raised a theatre and named it the Thalia theatre after her, and playhouses across Europe and America have taken her name ever since, so that the Muse of comedy still stands above the doors of the profession she invented. Her truest monument, though, needs no carving. Every sitcom set that tapes before a laughing audience, every stand-up stage, every village pantomime where the villain gets his comeuppance and the lovers end in an embrace, is a komos still marching, a revel that has refused for twenty-five centuries to go home. From the vintage roads of Greece to the screens of the modern world, Thalia remains the Flourishing, the laughter that proves a people is still growing.