We were thinking and talking about the popular notion of "scripture alone"* in one's faith and practice last night, and I've been working through various ways to properly express the problem. Just say that Friend X and I are members of the local Baptist congregation, and we're hanging out and studying the Bible (we're good members of the local Baptist congregation, thank you very much). Suppose that Friend X says to me, "you know, I've been reading Acts and 1 Corinthians here, and I really think that it's proper and right that our church services include periods of public prayer and prophecy by lay people, and as long as it's done decently and in order with appropriate interpretation, some of that prophecy and prayer will be spoken in unknown tongues." We'll have a nice chat about whether the norm in the Corinthian church was glossolalia or xenoglossy (essentially, whether the unknown tongues were human or non-human languages), and then by the end of it, I say to Friend X, "No, gifts like that passed away with the apostolic generation."
(You know that he and I would never really have this conversation, right?)
Here's the problem: Friend X and I have each offered a particular reading of Scripture. It doesn't make any sense to talk about one being "more scriptural" than the other, because we're both two people who worship with the same community, read the same scriptures, pray together, and bring that formation and our broader account of the Bible into our reading. We're each trying to make the most sense out of the biggest part of Scripture as well as we can, and we're good enough friends to assume good faith of one another. As "scripture only," evangelical protestants, we would be unable to appeal to any authority to adjudicate between these positions. There is no authority to declare either reading in or out, because there is not authority that can set boundaries on the reading of Scripture.
The Scripture itself does not tell me whether this disagreement is over core issues, or adiphora - is it something about which we can agree to disagree? The congregation cannot both engage in public prophecy in the manner of 1 Corinthians, and not do this. What authority can say to us, you must stay together as friends and fellow bible readers, or that you must walk apart?
Each one of us is holding onto a particular reading of scripture - an interpretive tradition.
In one sense, tradition is (as Tom Wright says) the history of the Church's Bible reading. It's a very long and quite diverse history, with people running around every which way. When we look for a Tradition (note the capitalization), we're asking the question, "In the long history of the Christian Church, which readings of Scripture have seemed to the broadest parts of the Church to be most faithful to the entire Biblical narrative, and most conducive to the growth in holiness and Christlikeness of the Church's members? In the broadest consensus of holy men and women in the Body of Christ through time and space, which readings of Scripture have been disastrous for the Christian life, and which have been a boon?
If appealing to the "Bible alone" were practically sufficient, we would not need to divide over diverse readings of scripture.
*I didn't use the popular Reformed phrase sola scriptura because I don't have any Calvin or Luther to hand, and it's not the Official Reformation Christian Doctrines® I'm disputing - we can go around all day about What the Reformers Really Meant (as if it matters) and never get back 'round to what real people in real churches really do to the Bible and to each other, and the entire discussion would bore me so badly, I would lose the will to exercise bladder control.