Since classical times, metaphor has been considered a figure of speech in which a concept is understood through the qualities of another. So, if we say “your
eyes are pearls”, we are describing the eyes as having the qualities of pearls:
those eyes condense the bluish, translucent, delicate and lustrous beauty of
pearls. Also since classical times, metaphor has been considered to belong to
the realm of rhetoric, that is, as an attribute of human imagination disconnected
from daily life. Simile and comparison are looser figures in which the
identification between both terms of the metaphor is not complete and retain
their independence. Thus, if I say “your eyes are like pearls”, I am suggesting
a way of thinking about the eyes that resembles pearls rather asserting that
both eyes and pearls are the same.

It has only been on the second half of the 20^th century that new approaches to metaphor, especially coming from developments in cognitive linguistics, have revived and transformed the way metaphor is defined and conceived. Cognitive approaches argue that metaphor is not merely an abstract device used by poets to embellish literature with disembodied imagination. First of all, cognitive linguisticians claim that metaphors do not occur as the manifestations of abstract, disembodied imagination in the Cartesian fashion. Rather, eminent cognitive theorists like George Lakoff and Mark Johnson proved in their seminal book Metaphors We Live By that metaphors are one of the many cognitive processes the mind uses to conceptualise, understand and interpret the world. They don’t deny that metaphor is a literary device but they conceive it as creative extensions and elaborations of metaphoric cognitive processes. In their book they present
evidence of how our everyday language is pervaded with metaphors. Cognitive
linguistician Zoltan Kovecses, in his book Metaphor in Culture: Universality and Variation, presents evidence of metaphors in different languages like Hungarian, Chinese, English or Zulu; for instance, Kovecses argues that Zulu and English languages have metaphors with similar source domains like “fire” to conceptualise anger though they are elaborated in a different manner.

Finally, recent developments in cognitive linguistics refer to metaphor as blending in what is called Blending Theory. According to this theory, metaphors do not consist solely of two elements, source and target; rather, they explain metaphor in more detail, as having four different stages in which inputs are processed and elaborated in the blending space. Proponents of this theory (Mark Turner and Gilles Fauconnier) link conceptual blending to the early stages of human development in the Palaeolithic, when human beings developed
an amazing capacity to innovate thanks to the mind’s capacity for conceptual
blending, through which humans were able to develop language and abstract/symbolic thought.