According to etymonline.com, the word plain (as used in e.g. explain or plain English) first appeared in the sense of "flat, smooth, even". Later, the sense of "explicit, clear, evident" appeared.
what is the metaphor involving flatness that points towards "explicit, clear, evident"?
Other words that refer to "explicit, clear, evident" have underlying metaphors that are known from texts since ancient times:
- lucid: dark room (non-seeing) is lightened up by a candle (seeing).
- clear: the sky goes from cloudy (confusion) to clear.
Are there also old texts which use plain ("flat, smooth") as a metaphor for "explicit, clear, evident"? Where does this transferred sense come from?
Some guesses (without attested texts):
- A turbulent sea surface as opposed to a still and smooth one? A metaphor for the mind.
- A bumpy landscape as opposed to a flat surface? A metaphor for ease, uniformity, and simplicity of moving ahead.
Also, some possibly related words: explicit (unfold) and unpack.