Are Paul's Letter's "Scripture"?

Someone asked me the following questions recently: 

Do you think that Paul wrote his letters to the church as Scripture? I mean they were letters. He quoted Scripture but do you think he was writing these letters to become church doctrine?

This is a great question, and the answer isn't simple. Let me break it down into 3 parts: Paul's view, the Apostles' view, and the Historical-Grammatical view.

Paul's view

Paul seemed to believe that most of the time he was writing by authority delegated by Jesus, not like he was writing letters dictated by God, but that he had been given a divine commission to expound on the Tanakh and the teachings of Jesus. Sometimes he spelled out that he was writing prophetically, as revelation from God. See 1 Corinthians 14:37 and Galatians 1:11-12. He also directed that his letters be circulated to some extent. See Colossians 4:16.

As a counterpoint, Paul also sometimes said he was writing advice that was not binding as a divine command. This is especially true in 1 Corinthians 7, but implied in personal directions to specific congregations. For example, Romans 14, 1 Timothy 5:23, and Titus 1:5.

The Apostles' view

Luke seemed to believe that Paul's speeches were important enough to record them in great detail in the book of Acts, but we really only have Peter's testimony regarding Paul's letters. 2 Peter 3:16 calls Paul's letters Scripture. Technically the word just means "writings", but the context clearly indicates that he meant divinely inspired writings, particularly the Tanakh.

The Historical-Grammatical view

Looking at Paul's letters from the outside, considering his expressions, manners of address, personal asides, and historical context, it seems pretty clear to me that Paul didn't think of his letters as Scripture on the same level as the Torah or Prophets. They were personal letters that contained divinely inspired instruction and prophecy, but weren't Law in the sense of Exodus or Prophecy in the sense of Isaiah. Each letter was addressed to a specific person or group of people and written to help with one or more specific problems.

You're familiar with the different classifications of Scripture: Law, History, Wisdom, Poetry (I think those 2 deserve separate categories), Prophecy, Gospels, Epistles, Apocalypse. OT History tells how the Law plays out in the world. Poetry tells how someone feels about how the Law plays out. Wisdom gives advice on how to live in a land governed by God's Law in the midst of a world corrupted by evil. Prophecy tells what will happen because people reject the Law and calls them back to it.

I think the Pauline Epistles and the General Epistles should be considered separate categories. Paul's letters were personal and situational, while the General Epistles were broadcasts, more consciously "Scripture", with general instruction on how to live according to the Law while in exile in a corrupted world. It seems to me that Paul's letters are blend of Epistle, Wisdom, and Prophecy. Sometimes he prophesied. Sometimes he rebuked. Sometimes he wrote about overarching spiritual principles that apply universally, but mostly he wrote divinely inspired advice into the specific circumstances of mostly-Gentile congregations trying to figure out how to live by this new set of standards in a hostile, pagan world.

Paul didn't have authority to change God's Law--not even Jesus could do that--but he had authority to explain it and to put it into the perspective of Gentile converts who didn't have the advantage of growing up in a Jewish culture that was infused with Torah. Everything he wrote was based solidly on the Torah, but it wasn't itself Torah. As long as we understand that his letters are mostly advice, not Law, then we can take the advice that he gave to Timothy or to the believers in Rome, study the Tanakh to ensure that we understand his advice within that greater context, and figure out how it applies in our own circumstances.

Paul's letters are also a valuable litmus test to see if someone is reading them only within the context of the whole Bible. If a person believes Paul had authority to nullify prior Scripture, then he will interpret him as being very antinomian. He will lean into one of the major doctrinal systems that reject the Law: dispensationalism, reform/covenant theology, or Catholicism/Orthodoxy. If he believes Paul only taught was already established by prior Scripture, then his letters take on a whole new character and depth.