Have you ever read Gaston Leroux's
Phantom of the Opera? I don't mean simply having watched the movie. The novel itself quite obviously pretends
not to be a novel. It's a while since I read it myself, but if I recall correctly: The author begins not in narrative but with assertions that everything in it is true and has been alluded to and alleged in various newspaper articles, and ends again by saying how he has gotten all this information and further asserts that he has found no less than the skeleton of the Phantom with the score for his masterwork
Don Juan Triumphant. In fact, when I read this novel for the first time in its entirety, I thought for a while that perhaps this Phantom was a real, historical figure!
Now, on text alone it is entirely obvious that the author wrote his work as history. If we went from there and said, however, that "The author wrote it as history, therefore it is history", we would be mistaken! The author may have wrote his work as history for every other reason: he wanted it to sound authentic, he wanted it to have dramatic power, he wanted to lend solemnity to veiled criticism of the powers of the day, he wanted to have fun. But he certainly would not intend for his work to be read as history, simply because he wrote it as history!
Not only that, the application of external evidence (in this case, we know the Phantom of the Opera simply didn't exist) does
not allow it to "mean whatever you want"! The external evidence will not allow me to read it as if it is history if I want it to be history. Even if I want it to be history, the evidence will not allow it to be. In that way external evidence places even more stringent criteria upon the interpretations of the Bible than text alone does. Believe me, there are days when I
want the Bible to be simply historical, to have a poor Adam and Eve tottering around the Middle East and biting the wrong fruit and landing us all in this mess. Life would be a lot simpler. But the evidence does not allow me to read the text any which way I want.
But alright. Let's say the rule is that "whenever a passage has any metaphorical element to it, then the whole passage must necessarily be metaphorical, instead of a scientific description of the world as we know it". Now, we will have to consider a lot of things in the Bible as metaphorical, for as Galileo said
Hence in expounding the Bible if one were always to confine oneself to the unadorned grammatical meaning, one might fall into error. Not only contradictions and propositions far from true might thus be made to appear in the Bible, but even grave heresies and follies. Thus it would be necessary to assign to God feet, hands ans eyes, as well as corporeal and human affections, such as anger, repentance, hatred, and sometimes even the forgetting of` things past and ignorance of those to come. These propositions uttered by the Holy Ghost were set down in that manner by the sacred scribes in order to accommodate them to the capacities, Of the common people, who are rude and unlearned. For the sake of those who deserve to be separated from the herd, it is necessary that wise expositors should produce the true senses of such passages, together with the special reasons for which they were set down in these words. This doctrine is so widespread and so definite with all theologians that it would be superfluous to adduce evidence for it.
I might ask, for example, did God literally speak? Does God have a literal larynx and lungs and literally expel sound from a literal mouth? If not, then every mention of "God said" in Genesis 1 is necessarily a metaphor.
Or was the Spirit of God literally hovering over the waters? God as Spirit cannot have a physical location, and so any mention of His hovering is a metaphor as well (not to mention the fact that the word is often translated "brooding" in more flexible translations, giving rise to a whole new dimension of metaphor).
Did the land literally produce vegetation? No less a literalist than
mark kennedy argues that the plain, obvious meaning of the text must be amended here and that the land produced seeds, not vegetation.
Did God literally exhale into man's nostrils the breath of life? And did God literally wait until He had created and commanded Adam of everything he should do before
realizing (as if God ever needed to realize anything!) that Adam had no helper?
Even with the one literary rule you put on the table, Genesis 1-3 are riddled through with metaphor for purely theological reasons, and hence deserved to be considered as metaphorical texts.