In another post, I mentioned one difference between exegesis of Scripture and of secular texts, stemming from the different role of the principle of charity in the case of an inerrant and a fallible text. There is also a second, perhaps less "hot", difference (not unrelated to the first) between interpreting the Word of God and interpreting works of philosophy (what I say will apply less in the case of poetry, fiction, etc.)
If I read Plato, I want to figure out what Plato thought about different issues, or what Plato thought at different times about different issues. The main reason I want to do this is because Plato was a pretty smart guy, and his views are likely to advance to search for truth. The focus, then, is on Plato. If Plato is silent in his extant works about some particular issue but I can find out from other historical sources what he thought about it, then apart from the accidental fact that in his case these other sources aren't all that reliable, this is about as good as reading it in his work. Thus, on the basis of the historical discussions of Plato's famous lecture on the Good, I think I know that it is a part of Plato's view that the Good has a knowable mathematical structure. I haven't read this claim in any of the texts, though the weird arithmetic towards the end of the Republic is confirmatory (I'm following Myles Burnyeat's interpretation here; I am no Plato scholar). Moreover, if I can infer from some text that Plato held p, then even if Plato never asserts p in the text, that still directly furthers my exegetical goal to know the mind of Plato. As a final thought experiment, if we found a lost work by Plato and were somehow able to authenticate it, this would automatically be just as good to us as the other works vis-a-vis the tasks of Plato interpretation, unless it were of inferior intellectual quality.
However, the case of the Scriptural works of St. Paul is different. There what we read as Christians is God's word. What Paul says is a direct guide to truth: what Paul asserts in Scripture is true. Knowing the mind of Paul is a means to the primary exegetical goal of knowing what the text, and hence the Holy Spirit, is teaching us. This is the reverse of the pattern we had in the case of Plato where knowing what the text is teaching us was a means to knowing whe mind of Plato.
This has implications. If we learned from reliable non-Scriptural sources that Paul believed p, this would not be just as good vis-a-vis our primary exegetical goal as reading p in the text of Paul's Scriptural texts. Learning that Paul believed p might well help us with exegesis of Scripture, but does not directly further the exegetical task of knowing God's word. What Paul teaches in Scripture is God's word. What Paul believes is a good thing to know, because Paul was smart, a good Christian and had good access to apostolic doctrine, but it does not have the authority that Scripture possesses. In the case of Plato, the text possesses no authority, and hence it is the mind of Plato we most want to know.
Likewise, if we can infer from a text that Paul believes some proposition, we should not treat that proposition on par with the propositions actually taught by Paul in the text. That proposition tells about the mind of Paul, but does not tell us directly about the mind of God who inspires Scripture. It is interesting what is on the mind of Paul. It may shed a light on what the meaning of the text in various places is, given that the meaning of a text is closely bound up with the expressive intentions of the author. But it lacks authority.
In particular, it follows that even if inerrance holds, there can be texts in Scripture from which we can infer that the author was ignorant of something or even believed a falsehood. For instance, consider the following pattern of reasoning that we employ in the exegesis of secular texts: "The author wrote that p. The only reason the author might have thought p was relevant in this context was because he believed q. Hence the author believed q." This is a perfectly fine piece of reasoning. But even if we can sometimes correctly deploy it in the case of Scripture, q will not have the authority of p. It is not what the human author believes, as such, that has authority, but what the human author teaches.
This observation, especially combined with Donnellan's famous distinction between the referential and attributive uses of language, can be a powerful tool for handling certain concerns about inerrance in cases where it is evident from Scripture that the author believed a false scientific claim. As long as the author did not assert the claim, or something that entails it, there is no difficulty.
Of course what St. Paul teaches is a subset of what he believes--the texts of Scripture are sincere.
A way to highlight the difference is what would happen if we found a new text by St. Paul. Unless we had good reason to include that text in the canon (e.g., we also found lots of early Church texts showing that the text was accepted as part of the canon; this is unlikely, since one would expect that if a text were considered canonical, it would have been copied a lot and hence it wouldn't have been lost). This would be a wonderful find for knowing the mind of Paul. But it would not have the kind of authority Scripture does. Though, in the special case of Paul, it might have some Apostolic authority (and in the special case of Peter, even infallible papal authority), but only if it was a text intended to bear authority.
However, I do not want to deny the fact that there are purposes for which we can read Scripture in the same was as we read Plato. Someone writing a book on the life and views of Paul of Tarsus will read St Paul's Scriptural works not as Scripture, but as reflections of the mind of Paul. Moreover, such work can be extremely illuminating for the exegesis of Scripture because of the at least partially constitutive role that the author's intentions play in the meaning of the text. But the goal of the primary sort of Biblical exegesis is to figure out what the author teaches in the text.
I want to also note that there are modes of reading Plato that are more interested in what the text says than in what Plato thinks. There is certainly room for that sort of reading, and it is important. In the case of works of fiction, this kind of reading is crucial. In the case of Plato it can still be very important. It is less important in the case of a less literary text, like Aristotle's texts. The line is not quite as clear-cut as I indicate above.
Let me end on the note there are other kinds of important and properly Scriptural exegesis besides this: there is allegorical, tropological and anagogical, for instance. These kinds are even less about the mind of Paul and even more about the mind of God. But these kinds rest on the foundation of what I have been talking about, which the Church Fathers called a discernment of the "literal" meaning of Scripture (certainly not to be understood as "literalistic").
