Steven Pinker: the presupposition of metaphor

Why must we refer so often to something else to say something specific about the here and now? In the Toronto Star article, Of Thought and Metaphor, Steven Pinker offers some insight on the importance of metaphor in speech. Near the end:

The boring explanation is thatusing metaphor is a quick-and-dirty way of expressing a new idea without the trouble of coining [notice the metaphor] and propagating a new word.

But that presupposes that the mind itself works metaphorically, that we see the abstract commonality between argument and war, between progress and motion. And it presupposes that the mind, at some level, must reason very concretely in order that these metaphors be understand and become contagious.

Pinker, author of the book Blank Slate: The Modern Denial of Human Nature, will be out with another, "The Stuff of Thought," (not available yet for pre-order) later in the year. He goes into more detail about his work in this interview at Gene Expressions, including a brief description of the forthcoming book.

Of great interest to me is the "presupposition" Pinker pursues: just what is that "concrete" process able to produce metaphor and language, the miracle of a child's first utterance as well as the soaring text of William Shakespeare?