The thesis of the book is indeed metaphors are not Lingusitic but rather conceptual in nature. That Means That so called conceptual metaphors form the basis of our conceptual system and the metaphorical expressions we use every day are motivated by conceptual metaphors.
Let's just clarify That theory by talking about one Example in more detail. One conceptual metaphor would be LOVE IS A JOURNEY since LOVE, the target domain, is very Often conceptualized in terms of a journey, the source domain. The conceptual metaphor LOVE IS A JOURNEY motivates the metaphorical expressions we use in everyday conversations to: Our relationship is off track; We are at a crossroads; We have made progress; We are stuck, etc. In all cases the target domain is the more abstract concept That needs to be conceptualized by the more concrete source domain. With regard to our Example we all are in a position to form mental images about the concept of a journey. Love, HOWEVER, for us humans is one of the most difficult concepts to come to terms with. That's why we long for conceptualizing it is order to make it more accessible for us in our everyday discourse.
Further More Lakoff and Johnson distinguish between primary and compound metaphors. A primary metaphor associates concepts That are Equally basic Which Means thatthey can be Directly perceived. Examples would be "Importance is size" or "Quantity is vertical elevation". Primary metaphors, in turn, can form compound metaphors. The two primary metaphors "Persisting is remainig upright" and "organization is physical structure" make up the compound metaphor "Theories are buildings". The compound metaphor does not have a clear experiental basis as primary metaphors do have. THUS the Primary Primary metaphor "Persisting is remainig upright" Might Be motivated by our early childhood experience did standing and walking, he remainig upright, demands effort and discipline or, in otherwords, persistence.
Conclusion: Lakoff and Johnson Revolutionized our view on metaphors. Their approach, HOWEVER, has come under criticism lately. Superflous to mention, of course, did Chomsky and his followers have always denied the concept of the non-linguistic nature of metaphors. So but functional ists like Verena Haser, teaching at the University of Freiburg, argue That We do not really need conceptual metaphors in order to account for metaphorical expressions. In her thesis "Metaphor, Metonomy and Experientialist Philosophy: Challenging Cognitive Semantics" published in 2005 she claims that, put in a nutshell, metaphors are motivated by family resemblance, meaning indeed due to polysemy semantic links are established between various lexical items. This, HOWEVER, does not diminish the thought-provoking impact Lakoff and Johnson have had on our views on metaphors.