The Argument:
1. If Ion is an exegete or explicator of Homer's poems, he must surely understand what the poet means, else he could not explain the poet's thoughts. This seemingly commonsensical point is asserted by Socrates at the start (530c1-5), and happily accepted by Ion.
Therefore: if Ion understands what the poet says about X, and judges that the poet speaks best about X, he must be in a position to assess other poets' pronouncements about the subject in question.
2. If Ion is the greatest of all Greek bards, he must know good poetry from bad. Homer is the best of all. He must be able to judge good poetry form bad. Ion agrees to this.
Plato’s conclusion: If you can knowledgeably (531e10) pick out a good speaker on a subject, you can also pick out the bad speaker on it, since the precondition of doing the former is that you have knowledge of the relevant subject matter. But this seems to contradict Ion's assertion that he can explain only Homer, not the other poets.
He only knows Homer, he can’t judge good from bad.
For the bard and the muse to invoke Homer can not be proven to be the best way to truth because there is no way to judge. It must be justified.
Furthermore, Homer himself must have understood well that about which he speaks.
2. It would seem that Homer claims to be wise, and that as his devoted bards must be claiming to be wise (532d6-e1).
In passage after passage, Homer pronounces on subjects that are the province of a specialized techne (art or skill), that is, a specialized branch of knowledge.
But neither the rhapsode nor Homer possesses knowledge of all (or indeed perhaps any) of those specialized branches (generalship, chariot making, medicine, navigation, divination, agriculture, fishing, horsemanship, cow herding, cithara playing, wool working, etc.).
So Ion, and by extension Homer, are faced with a series of unpalatable alternatives:
a. They could continue to defend the claim that they really do know the subjects about which they discourse — in the sense of possess the techne kai epistemeof them, i.e., a mastery of the subject matter. Yet if they do defend that claim they will be liable to examination by relevant experts.
b. They could admit that they do not know what they are talking about.
For Plato, this means that they must be held accountable. It is philosophy's mission to force them to give an account of themselves, and to examine its soundness.
This would mean that they are required to engage philosophy on its turf, just as Ion has somewhat reluctantly done. The legitimacy of that requirement is itself a point of contention, it is one aspect of the quarrel between philosophy and poetry.[8]
From Ion we get directly to Homer.
Ion claims to know good poetry from bad, Plato proves that he does not.
Ion claims knowledge from Homer, Plato proves he does not know.
Homer is held to be the Greek standard of truth. Plato shows that it is not.
Because Homer deceivers truth, what he teaches is true. Plato shows that it is not.
