Moreover, the comparison between written sources and visual representations demands further examination, since discrepancies are always likely to occur when comparing sources from different media (in this case, literature and visual arts). This topic has been thoroughly investigated by Lowenstam who enumerated a list of reasons why occasionally an image may ‘deviate’ from a known narrative: 
- the very difference of media may have caused difficulties in reproducing a narrative
- painters may have not known a traditional story
- the use of a traditional iconography led to departures from a known narrative
- the addition of labels (with names of epic characters) to a genre scene originated a composition where new details clashed with the older ones
- artists presented their own versions of myths
- artists combined different moments of a story in a single scene
- a different version of a story was followed or a combination of different sources
Even though these considerations are undoubtedly decisive in most cases, it has long been acknowledged that they have a major flaw in the assumption that paintings are necessarily related (and subjected) to a narrative, thereby excluding the option that a painter may introduce a new theme himself.  In this sense, the scene depicted by the Sosias Painter does not seem to fall into these categories or to cross some of them and consequently it proves to be a good example of issues that arise when approaching the topic of reception of the epic cycle in vase-paintings.
2. The Sosias’ cup 
However, difficulties arise because the episode represented on the cup, involving two main epic heroes, is not narrated in any extant literature.
Two interpretations have been proposed so far to tackle this issue. The most recent one,  argues that the painting alludes to the future events awaiting both heroes, by means of a tragic contrast with what is represented. Thus, the viewers would associate the scene where Patroclus is healed by Achilles to the fatal doom he is bound to, namely his death caused by Hector. Achilles’ attitude too would carry a tragic meaning: he is represented as capable of taking care of his friend, but will be absent when Patroclus is killed and, later on, he will be himself overcome by the fate and he will never come back from the battlefield. Though fascinating, this interpretation remains largely unprovable, since it relies on allusive processes that are very difficult to assess (especially in the case of episodes unknown to us, i.e. Achilles taking care of his wounded friend). It seems unlikely, then, that a painter created a scene ex nihilo, with the only purpose of alluding to a very different episode.
However, Patroclus’ participation in the Mysian campaign together with Achilles is attested by Pindar. In Olympian 9.70–75 Patroclus’ strength and courage are emphasised since he was the only one able to resist together with Achilles against Telephus’ attack. Shortly after, despite this achievement, Achilles warns Patroclus never to fight in a battle without him:
77 παραγορεῖτο μή ποτε
78 σφετέρας ἄτερθε ταξιοῦσθαι
79 δαμασιμβρότου αἰχμᾶς
It seems highly likely, thus, that the scene painted by the Sosias Painter must be considered in relation to the adventures of the two heroes narrated in the Cypria (or in a poem about the Mysian expedition) and that it stems out from episodes similar to what Pindar recalls.
3. Healing heroes in Greek mythological tradition
Jason and Achilles had a close relationship with Chiron and they both learned how to cure injuries, even though only sporadic references to their medical knowledge survive. As for Jason, the most ancient reference to his training by Chiron is attributed to Hesiod:
ὃν Χείρων ἔθρεψ᾽ἐνὶ Πηλίῳ ὑλήεντι.
Accordingly, Pindar groups Achilles, Jason and Asklepios together for having been raised by Chiron in Nemean 3.43–55 and the most detailed account about his youth is given in Pythian 4, in the words of Jason himself. When he comes in front of Pelias, he states that he aims at bringing there the ‘teachings of Chiron’ (φαμὶ διδασκαλίαν Χείρωνος οἴσειν, Pindar Pythian 4.102) and recalls that he was given to the centaur when he was a baby, he stayed at his cave for twenty years and apparently even his name, Jason, was bestowed to him by the centaur (Φὴρ δέ με θεῖος Ἰάσονα κικλήσκων προσηύδα, Pindar Pythian 4.119). This remark is generally interpreted as a pun on the etymology of Ἰάσων, deriving from ἰάομαι ‘to heal’, maybe implied also at Pythian 4.270 by the term ἰατήρ. 
The word-game on Jason’s name appears to have been quite transparent in antiquity, as the ancient comments to this passage suggest:
Other late accounts about the education of Jason report that he was taught medicine by Chiron:
Jason’s healing skills are not explicitly mentioned in any other literary account, but they were portrayed on a Corinthian crater, now in fragments (Figure 2):  the decoration shows Jason (iscr. Ειασον) laying his hands on Phineus’ eyes (iscr. Φινευς). Besides him there are Poludeukes (iscr. Πολυδ(ε)υκ(ε)ς), a woman perhaps labelled as Timandra (iscr. Τι.α.δρα) and a third figure interpreted as Kalais (iscr. [Κ]αλ[αις]). This scene does not recall any known narrative about the Argonauts, but, thanks to the inscriptions, it has been convincingly interpreted by M. Vojatzi as a depiction of a healing episode: Jason is curing Phineus’ blindness by laying hands on his eyes.  The story of Phineus regaining his sight is not preserved in any literary account, while the healing of his sons’ blindness was treated in the lost Phineus by Sophocles, where they were cured by Asclepius.  Even without a confirmation by literary sources, it may well be possible that in an ancient version of the myth Phineus himself was cured and that the healer was Jason (after all, the encounter of the Argonauts with the blind Phineus was a well-established part of the Argonauts’ journey).
As a healing hero, Jason may serve as a good parallel for Achilles. According to one legend concerning his youth, Achilles too had been raised by Chiron and his training consisted in using the javelin, running, and hunting, as described by Pindar (Nemean 3.44–49).  Euripides’ brief remarks focus only on the teaching of a moral behaviour (Iphigenia in Aulis 706 and 926–927), while Philostratus adds also music to the list (Heroicus 5.4–7). 
However, his ability to use medical herbs is more rarely recalled. The only instance in the Iliad occurs when the wounded Eurypilus asks Patroclus to cure him with the remedies that he learnt from Achilles, in turn educated by Chiron:
831 ἐσθλά, τά σε προτί φασιν Ἀχιλλῆος δεδιδάχθαι,
832 ὃν Χείρων ἐδίδαξε δικαιότατος Κενταύρων
In the Telephus’ episode the hero is again portrayed as a healer, even though under very specific circumstances. According to the summary of the Cypria preserved by Proclus (Chrestomathy 132), Telephus, king of Mysia, was severely injured in battle by Achilles, but afterwards, following an oracle, Achilles himself was requested to heal him. According to later sources, the content of the prophecy was that Telephus was destined to be the leader of the Trojan expedition and that he had to be cured by the same person that injured him. Therefore, other versions of this legend developed. In Hyginus’ account, Achilles refused to heal Telephus stating that he was ignorant about medicine, so the response was interpreted with reference to Achilles’ spear (as the effective ‘injurer’) and Telephus was cured with the rust scraped from it (Hyginus Fabulae 101).  A third version, then, states that Achilles used the rust from his sword for the medication (Apollodorus Epitome 3.20). Pliny the Elder recounts the same story, insisting on Achilles’ medical skills, when describing a plant called achilleos millefolium:
As in the case of Jason, it seems that Achilles’ healing knowledge was part of the mythological tradition, as a feature shared by heroes educated by Chiron, even though it is not much emphasised in the extant literary accounts.
One additional remark may help understand the depiction on the Sosias cup. Helping a friend wounded in fighting was not an uncommon feature in heroic narrations: medication scenes are described both in the Iliad and in the Odyssey  and a noteworthy example is offered by Sthenelus’ assistance to the wounded Diomedes, as narrated in the Iliad (5.95–113). Being struck by an arrow in his shoulder, Diomedes pauses briefly from the fighting and calls for Sthenelus’ help, but right after he pulls out the arrow, Diomedes comes back to the combat. The same two heroes are also depicted on a Chalcidian amphora showing them during a break from the battlefield (Figure 3),  and Sthenelus is shown bandaging Diomedes’ wounded finger. Besides them the other warriors, whose names are inscribed, are still fighting over the body of Achilles. The details of the depiction clearly refer to a different episode from the Iliadic one, even though the same two heroes are involved. Still, at least from a thematic point of view, both the narration in Book 5 of the Iliad and, mostly, the medication scene on the Chalcidian amphora are close parallels to the scene painted by Sosias.
It may be relevant to add that Diomedes and Sthenelus constitute a heroic duo that share many features with Achilles and Patroclus, namely that they participate together in heroic deeds and they develop a strong friendship bond, despite their different levels (because one is more prominent than the other—one is the main hero, the other one is his ‘assistant’, on which cf. infra).  A striking similarity between the two relationships was also underlined by Philostratus, active in the second century A.D., who mentions the special bond between Diomedes and Sthenelus, recalling that Homer did not value them equally and comparing them with Achilles and Patroclus:
Thus far, it may be argued that the portrayal of Achilles on the Sosias cup, though unprecedented in Greek iconography, is not in contrast with the mythical details presented above. The Sosias Painter probably knew about Achilles’ healing skills, and he created a composition that highlights this feature while easily reminding the viewer of the close friendship bond between the two heroes.
4. The friendship between Patroclus and Achilles
In the Homeric narrative, however, this relationship is not explicitly presented as an erotic one,  while later authors concentrated more on this point and by the fifth century the relationship between Achilles and Patroclus was unanimously considered homoerotic. Aeschylus focused on the love between the two heroes in the Myrmidons  and almost a century later Plato celebrated this relationship in the Symposium, presenting it as an example of the purest form of love, the one that is also rewarded by the gods:
Still in the fourth century B.C., the orator Aeschines mentions the relationship between Achilles and Patroclus as well, arguing that Homer did not present their love explicitly because it was unnecessary, since this feeling was nevertheless evident to the audience:
In Greek iconography the deep affection felt by Achilles for Patroclus is usually shown through his grief when his friend dies, but the two are very rarely represented together and both alive.  The Sosias Painter’s composition, thus, appears to be innovative not only because it focuses on a moment that is never represented (nor narrated in literature, as far as we know it), but also, from the iconographic point of view, because it is one of the few extant depictions of Achilles and Patroclus together.
Apparently, the age gap between Achilles and Patroclus was a debated issue in the antiquity. According to the Homeric tradition, Achilles was younger and stronger than Patroclus, as his father Menoetius reminded him:
787 πρεσβύτερος δὲ σύ ἐσσι: βίῃ δ᾽ ὅ γε πολλὸν ἀμείνων
As for their social and military roles, consequently, their age is not considered and Achilles is the main hero, while Patroclus is his θεράπων.  In later times, however, their age was probably adapted to fit their social roles and thus Patroclus (inferior in strength and nobility to Achilles) was considered younger. Aeschylus, for example, considered Achilles as the older one and he was criticised by Plato (Symposium 179e–180b), for doing so.