It is not science, we know what meant by "Scripture", Paul was talking to a believer like him, both knew what meant by that word. Do you really think that Paul meant all holy books in the world are the "Scripture" ?!
I'm asking you. You're the one saying that St Paul said that "all Scripture" is inspired. Which Scripture?
You claim that St Paul must have meant the Old Testament. Very good. So we know that the OT is inspired because St Paul said so.
That still leaves the New Testament. How do we know the NT is Scripture? Obviously St Paul wasn't talking about the NT if it didn't yet exist. And since it didn't yet exist, can we really trust St Paul when he talks about the OT? On what basis?
Because he's a Jew and the Jews accepted the OT? Jews don't accept the NT, so maybe we shouldn't either...apparently they are able to tell us for certain that some books are Scripture, but they definitely agree the NT is not.
Scripture doesn't tell us about the Bible as we know it. There are books where it says "Thus says the LORD", or "All Scripture is inspired", or "Blessed is the man who hears the words of this book", but those have to do with the individual books, not the entire collection.
So we're back to the original question. You say "we know what (is) meant by 'Scripture'". How? How do we know that?