What does the rule that "Scripture interprets Scripture" mean and where does it fit into a larger picture of the field of hermeneutics? What hermeneutical approaches use this as a guiding principle?
It simply means that the scriptures must harmonize. The orthodox Christian view of the Bible is that it is not in error and does not contradict itself. Therefore when trying to examine one passage we must approach it with an eye to what the whole Bible says about that topic. For example, let's take divorce.
“Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery."
While this is certainly clear from a plain reading, this is a controversial topic. You wouldn't see that from here, would you? When we look at what the Bible says about divorce in other places you can see why there is a dispute within the church on the topic.
When a man takes a wife and marries her, if then she finds no favor in his eyes because he has found some indecency in her, and he writes her a certificate of divorce and puts it in her hand and sends her out of his house, and she departs out of his house, 2 and if she goes and becomes another man’s wife, 3 and the latter man hates her and writes her a certificate of divorce and puts it in her hand and sends her out of his house, or if the latter man dies, who took her to be his wife, 4 then her former husband, who sent her away, may not take her again to be his wife, after she has been defiled, for that is an abomination before the LORD. And you shall not bring sin upon the land that the LORD your God is giving you for an inheritance.
It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ 32 But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
And Pharisees came up to him and tested him by asking, “Is it lawful to divorce one’s wife for any cause?” 4 He answered, “Have you not read that he who created them from the beginning made them male and female, 5 and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’? 6 So they are no longer two but one flesh. What therefore God has joined together, let not man separate.” 7 They said to him, “Why then did Moses command one to give a certificate of divorce and to send her away?” 8 He said to them, “Because of your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. 9 And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.” 10 The disciples said to him, “If such is the case of a man with his wife, it is better not to marry.” 11 But he said to them, “Not everyone can receive this saying, but only those to whom it is given. 12 For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. Let the one who is able to receive this receive it.”
This example is good because we can see a place where Jesus actually gives commentary and builds upon the OT. The key to this idea of "scripture interpreting scripture" is that you cannot look at one passage or verse in a vacuum. To earnestly understand what the Bible (and therefore God) says on a subject we must look at the whole of the book.
"Scripture interprets Scripture" is the principle of Regula Fidei from the perspective of Sola Scriptura. However, it's also a mind-set that is used through many hermeneutic appraoches.
Sola Scriptura/Regula Fidei
The idea behind Sola Scriptura is that the Bible is complete and sufficient for all knowledge of salvation and holiness. From this doctrine, the Bible should be used for judging faith and soundness of doctrine and practices; it is, therefore, the "Rule of Faith", or measure by which we judge something.
"We believe that the only rule and standard by which all dogmas and all doctors are to be weighed and judged, is nothing else but the prophetic and apostolic writings of the Old and New Testaments"
(Form. Concordiae, 1577)
Scripture interprets Scripture
From this idea that scripture alone was sufficient came the idea that we should interpret scripture using other scripture. Since the Bible alone was the sole measure for judging faith and practice, it should be used as a measure to understand other parts of scripture.
As a Hermeneutic Approach
As a hermeneutic approach, "Scripture interprets Scripture" is the idea that we should read a passage in the light of the entire Bible. It also states that we should interpret confusing passages based on clear passages.
"Scripture interprets Scripture" is, in itself, a hermeneutic approach. However, it's also a mind-set used by other hermeneutic approaches.
Other Hermeneutic Approaches
This idea that scripture can be used to gain insight into other passages in the Bible is found through many hermeneutic approaches.
Sensus Plenior is one example that uses "Scripture interprets Scripture" as a general guiding rule. Theological Analysis is a technique that shows that theology must harmonize throughout all the scriptures (so that one small passage doesn't create doctrine).
The idea that we can use scripture to interpret other scripture is a common theme throughout Hermeneutics. From the perspective of Sola Scriptura, it is, by itself, a hermeneutic principle.
Ultimately, it is simply the idea that we can use scripture to shed light on other passages within the Bible.
In sensus plenior it means that all the answers to the riddles are found within scripture.
Those who subscribe to the ONLY literal-historical hermeneutic are not permitted by that hermeneutic to see the "mystery hidden from the beginning" and therefore attribute any interpretation from sensus plenior as eisegesis without actually examining the methods by which an interpretation is exegeted from scripture.
When one simple rule is applied --that a metaphor must be the same everywhere -- one resolves the meaning of the metaphor by comparing it's use everywhere as in solving a crossword puzzle.
But if we use the rule:
Jesus says we should let our light shine before men, yet we should not let our left hand know what our right hand is doing. There are at least twelve different interpretations of this "apparent contradiction" by people using the literal-historic methods.
Jesus says Goats are on the left and sheep are on the right. So we should do the works of God for which we were created, but we should not let our 'goat nature', the flesh, interfere with the spiritual works we do. We should not be claiming credit for God's work as Nebuchadnezzar did.
So the Ninevites who did not know the left from the right, did not know the flesh from the spirit.
The sword-word (in Eglun and Ehud) was pulled from the right (spiritual) thigh-will with the left-flesh hand-works. God's spiritual will was accomplished by men working in the flesh.
The reason those using solely the literal-historical have so many differing interpretations is that they are not constrained by such a strict rule. They are free to let a metaphor have any meaning, and to push forth their interpretation using Greek debate and rhetoric.
In actuality, the meaning of 'literal interpretation' is that a writing is interpreted in it's proper literary genre. Poetry is interpreted as poetry, history as history. The claim of sensus plenior is that the genre is prophet riddle: It has a literal-historical meaning AND a prophetic meaning verifiably retrievable hidden within.
Therefore, technically, IF the sensus plenior of scripture exists, it's exegesis is a literal interpretation.