St. Augustine and Hermeneutics - Distinguishing Literal and Figurative

St. Augustine wrote On Christian Doctrine as a guide to interpreting the Bible. Fundamental to this is of course his insistence that Scripture has both literal and a figurative meanings; the literal meaning intended by the human author is not at all the only significance of any passage, because God is the “real” author of the Bible. How then do we know when to take a passage figuratively rather than literally?

In the first place, then, we must show the way to find out whether a phrase is literal or figurative. And the way is certainly as follows: Whatever there is in the word of God that cannot, when taken literally, be referred either to purity of life or soundness of doctrine, you may set down as figurative. [III.x.14]

This tells us more than just that single principle, actually. He also hereby tells us something of his understanding of what exactly the purpose of the Bible is: that is, it is to instruct us concerning how to live and what to believe. This being the case, then, any passage that doesn’t literally have something to do with either of those goals must be interpreted figuratively. The human author may not have intended this, but God must have.