ΣΩ. Ἐστὸν δή τινε δύο, τὸ μὲν αὐτὸ καθ’ αὑτό, τὸ δ’ ἀεὶ ἐφιέμενον ἄλλου.Given the classification to come between things that are geneseis and things that are ousiai, which is also meant to map a distinction between things that are for-the-sake-of something else and those for whose sake are other things (such as how ship-building stands to a ship) we should expect the relationship of beloved : lover or, to use the standard Greek terms, erōmenos : erastēs to function analogously to that of ousia : genesis. Pleasure will be assigned to the class of geneseis. Most importantly, therefore, it is the lover who is aligned with pleasure and ‘becoming’ while the beloved is aligned with a completion or goal and with ‘being’.
ΠΡΩ. Πῶς τούτω καὶ τίνε λέγεις;
ΣΩ. Τὸ μὲν σεμνότατον ἀεὶ πεφυκός, τὸ δ’ ἐλλιπὲς ἐκείνου.
ΠΡΩ. Λέγ’ ἔτι σαφέστερον. ΣΩ. Παιδικά που καλὰ καὶ ἀγαθὰ τεθεωρήκαμεν ἅμα καὶ ἐραστὰς ἀνδρείους αὐτῶν. ΠΡΩ. Σφόδρα γε. ΣΩ Τούτοις τοίνυν ἐοικότα δυοῖν οὖσι δύο ἄλλα ζήτει κατὰ πάνθ’ ὅσα λέγομεν εἶναι.
Soc. Let there be this pair: what is itself, by itself, and what is always aiming at something else. Prot. What are these two you are talking about and what are they like? Soc. The one is always by nature the most holy and the other lacks it. Prot. Be clearer still, please. Soc. I suppose we have seen beautiful and good young boys together with their brave lovers. Prot. Certainly. Soc. Then now look for another pair of things that are like these two in all the ways we are mentioning.
So I wondered whether that is indeed how we are supposed to interpret the erotic analogy. And this is as far as I have got. That the correct alignment is lover-genesis beloved-ousia can be supported by a variety of notions which build on a common, albeit perhaps idealised, picture of the lover–beloved relationship and also various conceptions of the lover–beloved relationship which can be found either in the surrounding context of the Philebus or elsewhere in Platonic texts:
1. It is the beloved who is here described in terms which refer to his beauty and goodness.
When Socrates finally explains the ousia–genesis distinction in terms of the value of the members of the two classes, he insists that ‘that for the sake of which something comes to be’ should be put in the class (moira) of goods while pleasure, if it is a kind of coming-to-be, ought to be placed in a different class and is therefore not a good (54c9–d3).
2. The beloved is the object of the lover’s aims and desires. It is ‘for the sake of’ the beloved that the lover undertakes various tasks, performs various acts and so on.
3. It is the lover, not the beloved, who feels desire. And desire is often figured as a kind of lack of absence. So in this sense, qua lover he is lacking.
Lack or deficiency is a characteristic of the genesis class of things (54d6–7) and is made a defining feature of the lover by Plato most obviously in the Symposium.
4. It is the lover, not the beloved, who is generally understood to take pleasure in the relationship.
This is not such a clear case. See, for example, Phaedrus 240d. To be sure, the context here is a speech in which the aim is to persuade the young man that he is better off taking up with someone who is not his lover, but the rhetorical tropes must be to some extent plausible to a general Athenian audience for the speech to be effective. Nevertheless, despite some exceptional cases, which are often intended as grotesque inversions of the norm, it is generally speaking the erastēs who is considered to take pleasure in any intercourse.
So that is as far as I have got so far. My next port of call is going to be Aristotle's analysis of philia and pleasure which also refers to these erotic relationships.