I shall address in this post and the next another common misunderstanding of the Affinity Argument, that it is an 'argument from analogy'.
If it is, then the criticism then proceeds: analogies are persuasive but not demonstrative; they can even be misleading; and Simmias' criticism shows how easy it is to devise an analogy which seems to support the opposite conclusion, that the soul is perishable.
Some lecture notes posted online by Sally Haslanger illustrate well this approach:
Bostock reaches a similar conclusion: "Fairly obviously," he says, "Plato does not himself think that the conclusion strictly follows from the premises, but knows well enough that analogies can never count as conclusive proofs; from the fact that the soul is like the forms (or like the gods) in one way, it will never follow that it is like them in other ways too. For all that, analogies can quite often be persuasive, especially if they are full and detailed. But the present analogies really are not: on the contrary, there appear to be equally good analogies pointing in the opposite direction, as Plato himself goes on to point out" (120).
II. THE AFFINITY ARGUMENT
(a) The world consists of two sorts of things: the things which are divine, intelligible, uniform, indissoluble, deathless (eternal), [invisible, incomposite] and the things that are not. (Arewe supposed to get these conclusions from recollection argument?)
(b) The soul is more like the former in the following ways (this is seen especially if we consider the soul of the philosopher which has been purified of bodily influences):
(i) it is invisible
(ii) it is closer to unvarying than constantly varying
(iii) it rules the body and so is more divine
(c) So the soul is plausibly one of the incomposite, indissoluble, deathless things.
The analogy is weak: we don't get obvious similarity of the soul in the crucial respects.
In particular, there are things which are invisible and incorporeal (the harmony), and possibly incomposite (material elements?), which aren't among the divine class of things Socrates mentions.
Yet what is the evidence that Plato does not think that the conclusion of the Affinity Argument 'strictly' (?) follows from the premises? Or that he acknowledged that there are 'equally good analogies pointing in the opposite direction'? (Doesn't he deny that Simmias' comparison is sound?)
In any case, these criticisms are based on a faulty appreciation of the Affinity Argument, which is not an 'argument from analogy' at all.
What is it, then? I'll explain tomorrow.