I went to a Sunday morning service in a university church. A young minister stood to preach from Mark 4. "The sower sows the seed," he told us, "and we see in the passage that the seed stands for the Word of God." So far so good. We were looking at the Bible. "But what is the Word of God?" he continued. "The Word of God is the intrinsic power of education to bring out the good in all people."

Oh dear.

Its a particularly clear example of someone who genuinely wants to look at the Bible, but when it comes to establishing meaning and application immediately jumps outside of the Bible for his paradigm for doing so. He assumed that the meaning of the text was not to be found in the text or in the Bible as a whole, but in certain assumptions about the meaning of "word" and about education in his university situation. I think this was a sincere young man, doing his best, but his assumptions were faulty.

In a comment on a previous post Adrian Reynolds notes the primary hermeneutic principle is that scripture interprets scripture. (Come and write some more on this with me Adrian). That is we go first to the rest of the Bible to tell us what a passage means before we ever bring in external material. If we don't then we let the world tell the Word of God what it should mean. Here are two examples (there are others):

A student said to me "if the Bible says one thing and I strongly feel the opposite, surely integrity demands that I can only go on my feelings. To do otherwise is to be untrue to myself." In this case feelings determine the application of the text
I frequently hear people say "parts of the Bible are written to a particular culture with cultural values that are no longer relevant to our culture. We need to change those bits to bring application that is more relevant to culture today." There is a long discussion to be had on this one, but what they are basically saying is that our culture establishes meaning and application of the text. Culture is primary over scripture. The difficulty is that you can make that case about the WHOLE Bible. Most commonly I hear this case made to justify taking a particular stance on why certain texts that seem to speak on gender issues and authority in the church should no longer be considered germane to contemporary church life. But those who make the case would normally be reluctant to conclude that because our culture justifies greed or sex outside of marriage that texts that speak to those matters should be relativised away

In both cases something other than scripture is held up as the lens through which scripture must be interpreted. In both cases scripture is captured within the gravitational pull of existing agendas. We make it say what we want it to say. It doesn't have to be deliberate, as in the case of the Mark 4 preacher. We can absolutise the bits we want and relativise away the bits we think are less relevant as a consequence of invisible assumptions about the act of doing hermeneutics.

Whole libraries are filled with books on how to let scripture interpret scripture. Here are a very few key principles:

The Bible as a whole has a plot line. If we don't know what it is we will cherry pick it as a collection of wise sayings, taking what we think we understand and ignoring the rest
Jesus is the goal, end and fulfilment of the plot line of scripture. If we don't see how a particular part of scripture relates to Jesus as the fulfilment of all God's plans an purposes then we won't let scripture interpret scripture. The primary context in which to consider any text of the Bible is the christological context: what is its relation to Jesus?
The surrounding verses, chapters and book tell us what a particular verse means. This is simply the way you read books. It is logical, coherent and sensible
Are there textual connections, references or allusions to other parts of the Bible that bear on what a passage means

In all these principles we are asking "what is the biblical rationale for this verse / command / passage / promise being here? In what way is this part of the Bible tied to biblical truth as a whole?"

At the end of the day the best principle for learning to interpret scripture by scripture is to read the Bible. All of it. Regularly.