The dividing line between nymphs such as Nereids and goddesses is both thin and flexible. According to some, Thetis (Greek Θέτις) was merely the senior of the Nereids, but others rate her a goddess in her own right. She’s not best-known in myths as a goddess, either, but as the mother of Achilles, greatest of the Greek warriors in the Trojan War. But in that she was also part of the cause.

Peleus, king of the unpronounceable Phthia, either bound Thetis or raped her, forcing her to marry him. Their wedding was celebrated with a great feast on Mount Pelion attended by most of the gods. The happy couple were given many gifts by the gods, but one, Eris the goddess of discord, had not been invited. As an act of spite at her exclusion, she threw a golden apple ‘of discord’ into the middle of the goddesses, to be given as a reward to ‘the fairest’. This in turn set up the Judgement of Paris, and led to the Trojan War.

Paintings of the wedding are lavish, filled with figures, and have a high flesh content. Among my favourites are the following.

Cornelis van Haarlem’s The Wedding of Peleus and Thetis from 1593 segregates the deities into a separate feast in a sacred grove on the left. There is, as yet, no sign of discord among them, nor of any golden apple. Some of the gods are still among the other guests in the foreground, including Pan (near his pipes, at the left) and Mercury, with his winged hat and caduceus at the right. They seem to be having a good time, and there’s ample choice as to which of the nude women might be the bride.

Joachim Wtewael’s undated painting of The Wedding of Peleus and Thetis is great fun, with its aerial band, and numerous glimpses of deities behaving badly. I think that I can also spot Eris, about to sow her apple of discord into their midst: she is in mid-air to the left of centre, the apple held out in her right hand. As to the bride? Again, it seems like anyone’s guess.

Hendrick van Balen and Jan Brueghel the Elder combined their skills to paint The Wedding of Thetis and Peleus together in about 1630. Here it’s the innumerable putti who seem to be running riot, and there is no sign of Eris or her golden apple, as far as I can see.

This is Jacob Jordaens’ The Golden Apple of Discord from 1633, based on a brilliant oil sketch by Rubens. The facially discordant Eris, seen in midair behind the deities, has just made her gift of the golden apple, which is at the centre of the grasping hands, above the table.

At the left, Minerva (Pallas Athene) reaches forward for it. In front of her, Venus, her son Cupid at her knee, points to herself as the goddess most deserving of the apple. On the other side of the table, Juno reaches her hand out for it too.

It’s the most modern version, painted by Edward Burne-Jones as The Feast of Peleus in 1872-81, which sticks most closely to the story. In a composition based on classical representations of the Last Supper, he brings Eris in at the far right, her golden apple still concealed. Every head has turned towards her, apart from that of the centaur behind her right wing. Even the three Fates, in the left foreground, have paused momentarily in their work.

Burne-Jones has gone further than simply composing this as a Last Supper, and the figure of Jupiter in the centre, holding a thunderbolt, is overtly Christ-like. I suspect that must have resulted in quite a storm at the time.

Long after Ovid’s rather vague account of how Achilles died, a new myth was invented, probably by the Roman poet Statius (who appears in Dante’s Divine Comedy), to explain his vulnerability and death from an arrow shot by Paris. When Achilles was a young child, his mother Thetis immersed him in the water of the river Styx, to make him invulnerable. However, she had to hold him by part of his body, the left heel, which was therefore left as his only weakness – hence, his Achilles Heel.

Rubens included this oil sketch in his Achilles series, showing Thetis Dipping the Infant Achilles into the River Styx (1630-35). This is seen taking place in the foreground, while in the middle distance Charon is seen ferrying the dead across the River Styx into the Underworld. Rubens complies with Statius’ story in making Achilles’ left heel the one left vulnerable.

Nearly thirty years after Rubens’ death, Jan-Erasmus Quellinus painted his version of Thetis Dips Achilles in a Vase with Water from the Styx (1668). It is set not on the bank of the River Styx, but at a temple, where Achilles undergoes a baptismal procedure in a a huge pot, at the lower left. Thetis appears to be holding the infant, who is almost completely immersed, by his left foot, again in compliance with Statius.

I suspect that Quellinus has engaged in a little deliberate Christianisation of this myth, which may also have made it more familiar to those who saw it, of course.

Antoine Borel’s more traditional account of Thetis Immerses Her Son Achilles in Water of the River Styx was painted at least a century later, in the late eighteenth century, and again has Thetis hold Achilles by his left foot.

Unusually for Rubens, though, his paintings of the death of Achilles show the arrow transfixing his right foot, not the left. That was a necessity by virtue of its composition, although Rubens could just as easily have reversed his drawing to achieve consistency with this detail.

Thetis appears again in Homer’s Iliad, at the height of the Trojan War, when Achilles takes Briseis as a prize of war. At the start of the account in the Iliad, Agamemnon, leader of the Greek forces, has decided that he will take Briseis for himself. Achilles resents this, and withdraws from involvement in the fighting.

In Achilles’ absence, his close friend Patroclus leads a battle in which he wears Achilles’ armour. Patroclus is killed there, and Achilles laments over his corpse. His mother Thetis visits Achilles during this, to console him in his grief, and promises to return with impregnable armour forged by the god Vulcan (Hephaestus).

When she does return with the shield and armour, Achilles is still lamenting over Patroclus, worried now that his friend’s body will decay if he returns to battle. Thetis protects the body of Patroclus with ambrosia and nectar, enabling Achilles to return to battle and kill Hector, the leading warrior of Troy.

Benjamin West completed two paintings of Thetis Bringing the Armour to Achilles. The first version above, painted in 1804 and now in Los Angeles, is the tighter composition and shows some influence, perhaps, by the more neo-classical narrative works of David.

As with most other depictions of this story, West shows Achilles with the body of Patroclus, and Thetis presenting the armour of Vulcan/Hephaestus to her son. He follows tradition by showing each of them largely unclad, with theatrical gestures and expansive sweeps of their arms. Although the underlying message might have had great contemporary relevance, West has resorted to traditional narrative technique and style.

Two years later, his second version of Thetis Bringing the Armour to Achilles (1806) has reversed the arrangement of the three principal figures, and added extras at the left and right (in the murky shadows) whose gestures are so theatrical as to now appear comical. This version also lacks the Davidian touch, and appears more like a narrative work from a century earlier, perhaps.

In 1866, the young history painter Henri Regnault won the Prix de Rome with his Thetis Bringing Achilles the Weapons Forged by Vulcan. This set title was taken from a famous painting, generally known as Thetis Receiving the Weapons of Achilles from Hephaestus, by Anthony van Dyck, from 1630-32. Here, the bare-breasted Thetis brings those impregnable weapons to Achilles. Among them is the famous decorated Shield of Achilles. (I apologise for the image’s small and fixed size.)

Thetis doesn’t usually appear in paintings of the death of Achilles, though, with one notable exception: Henry Fuseli’s Thetis Lamenting the Death of Achilles from 1780.

In the foreground, Achilles’ body lies like a fallen statue on his shield, his great spear by his left side. There is no sign of any wound, arrow, or injury. At the water’s edge, his mother Thetis is waving her arms in lament for her dead son. Another deity is flying past in the distance, and is seen white against the dark and funereal sea and sky.

Maybe Thetis was the goddess of mothers of heroes.