Mirror of the Forms

I earlier asked about the meaning of the following sentence in the Phaedo:

e)rrh/qh ga/r pou ou(/twj h(mw=n ei)=nai h( yuxh\ kai\ pri\n ei)j sw=ma a)fike/sqai, w(/sper au)th=j e)stin h( ou)si/a e)/xousa th\n e)pwnumi/an th\n tou= “o(\ e)/stin”

Here is how I think it should be understood:

For the soul of a human being (h(mw=n) was said, I take it, to exist even before it enters the body. This was said on the same grounds as that (ou(/twj ... w(/sper...) in the soul (au)th=j) there exists that sort of existence (e)stin h( ou)si/a) which has a derivative name (e)/xousa th\n e)pwnumi/an) that is derived from (th\n tou= ) that which is.

Remarks:

1. 'soul of a human being ...enters the body': 'human being' for h(mw=n, since the quoted expression seems to stand proxy here for ones that Plato uses elsewhere, when he speaks of the soul 'being in the form of a human being' (en anthrwpou eidei, 76c8) or 'entering a human sort of body' (eis anthrwpeion swma aphikesthai, 77b8).
   
2. 'This was said on the same grounds as': because what follows involves no reported speech. Apparently Simmias now wants to affirm for his own part the relevant grounds for the earlier conclusion that he reports.
3. 'in the soul': literally, the ou)si/a belongs to the soul; but this is the more natural way of putting it--the ou)si/a is within the soul as belonging to it. (This is not 'impossibly harsh' because it seems that Plato wishes to construct a parallel in syntax, which serves to highlight the parallelism of philosophical grounds: h(mw=n ei)=nai h( yuxh\ ... au)th=j e)stin h( ou)si/a...)
   
4. 'that sort of existence': ou)si/a is used earlier in the dialogue for a sort or class of existing things (and see 76c9 for 'that sort of ou)si/a').
5. 'that which is': Rowe is probably correct that this is a schematic expression. Unpacked, what I think it suggests is that, for each Form, there is something in the soul which exists and derives its name from it.
6. 'a derivative name': this would not exclude many things' in the soul having such derivative names, cf. the singular used for two opposites at 103b8.
   

I'll say tomorrow why I think that this is important for our understanding of the Recollection Argument. But here is a brief indication.

In the Recollection Argument, Socrates has his interlocutors agree on two occasions that the following claims stand and fall together: (i) the Forms exist; (ii) the soul pre-exists the body. This puzzles commentators, because they take standing and falling together to be the same as logical equivalence (co-implication), yet, although Socrates gives an argument from (i) to (ii), he doesn't give an argument from (ii) to (i). (Nor does it seem that such an argument is possible.)

The sentence translated above is clearly meant to refer back to that earlier agreement (although it reflects also, surely, Simmias' appreciation of the likeness of the soul to the Forms, which was emphasized in the intervening Affinity Argument). But if it does so refer back, then, it becomes clear that Plato understands the core idea of the Recollection Argument to be something like this: that there are things in the soul (innate ideas, if you will) which admit of being interpreted as imprints of the Forms (and in doing so we regard our names for them to be names derived from the Forms), but to interpret them thus, and to say that the soul pre-exists the body, is to affirm one and the same thing.

It's not that (i) and (ii) imply each other, but that these are two aspects of interpreting the same thing in a certain way.