Metaphor is ...

The extremes can be extended along their natural axes; but to extend something in the middle means pushing it along the z axis, into metaphor. That's what metaphor is, in a sense; this z axis poking three-dimensionally out at right angles to the 2D of ordinary signification. For example:

Too hot, too cold, is literal; but too tepid is metaphorical.
Too clever, too stupid ... too average.
Too expensive, too cheap ... too reasonably priced.

In each case the third term means in a different way to the first two items.