Only the original authographs were inspired-inerrant. In our translations there are some scribal errors that have occurred over the centuries. I have no problem using translations.
Ok, I don’t want to get into the differences in translations too much (although it can be a problem) but basically only the original writings (which we don’t have) were inspired-inerrant, but accurate translations and transcriptions would still provide a foundation for Sola Scriptura.
So, from that starting point – we know what Sola Scriptura is, and we know what we mean by the Scriptures.
For Catholics, the Scriptures are inspired and inerrant. God provided them to the Church to preserve the teachings of Christ and the foundation of Christian belief.
But we would also believe that even if all the Scriptures had been destroyed in the early days of the Church (as they almost were by Roman authorities), that Christianity would still survive and retain the true teaching of Christ.
We start by noticing that the early Christian communities did not have a canonized New Testament, but they relied on oral tradition to preserve the Gospel stories taught by Christ. For them, the written word could not have been the sole authority.
It was the apostolic founders of the Churches (which still exist today) which provided the authority and the teachings – they eventually preserved and canonzed the Bible and gave it to future generations.
We trust their decisions because they were protected by God in the promise made by Christ to His Church.
This one of those arguments – the Bible cannot validate itself as being an inspired text. Some other authority has to tell us that the books are inspired.
Some will say “God tells us” that the books are inspired, but I don’t think that every believer reads all the texts and decides for himself if the books are inspired or not.
We trust the Church – through the Conciliar decisions on the canon of Scripture. That’s how we know which books are inspired.