Diagrams and Metaphors

For Peirce diagrams are part of triad of icons that contains images on the one side and metaphors on the other.  But if he doesn't say all that much about diagrams, he says even less about metaphors. In that light, however, James C. Scott gives an interesting example in Seeing Like a State.

When the first European settlers in North America were wondering when and how to plant New World cultivars, such as maize, they turned to the local knowledge of the Native American neighbors for help. They were told by Squanto, according to one legend (Chief Massasoit, according to another), to plant corn when the oak leaves were the size of a squirrel's ear. [p. 311]

He notes with regard to this advice:

As a rule of thumb, it was a nearly foolproof formula for avoiding a frost. [p. 312]

And comparing it to a specific date or calendar event in an almanac Scott notes that this means is transferable to different latitudes and longitudes (wherever there are oak tree and squirrels) and is more efficient (as any specific date would have to be set more conservatively).  What strikes me is that it is a metaphor, a comparison of two systems whose diagrammatic complexity is ignored in favor of characteristics they share. Several hypotheses suggest themselves from this.

1. Rules of thumb should generally be interpreted as metaphors.
    
2. Rules of thumb (metaphors) generally come first and the diagrams later. Euclid's geometry, for example, was not so much abstractions directly from nature, but abstractions that could incorporate the geometric rules of thumb already in use (Euclid's Window).
    
3. In using language, following Peirce, we go from the object to a sign (in name only) to an interpretant. In this way we can talk, endlessly it seems, without really knowing what we are talking about.
    
4. Thinking diagrammatically is the effort of going back to the sign and laying out an analogous system that produces that interpretant, and others, from internally generated consequences.