How deep does metaphor go?

The most important claim we have made so far is that metaphor is not just a matter of language that is, of mere words. We shall argue that, on the contrary, human thought processes are largely metaphorical. This is what we mean when we say that the human conceptual system is metaphorically structured and defined.—Metaphors We Live By, page 6

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In other words, pretty deep. Extremely deep. (And even that wording of "deep" is a metaphor...)
</idle musing>