That’s how linguist Guy Deutscher characterizes all language. Prospero’s R.L.G. elaborates:
Most of the time we do not realise that nearly every word that comes out of our mouths has made some kind of jump from older, concrete meanings to the ones we use today. This process is simple language change. Yesterday’s metaphors become so common that today we don’t process them as metaphors at all. Primal man, in inventing language, would have only used concrete terms he could stub his toe on, like tree and rock. So tree is not a metaphor, and rock is not a metaphor. As far back as the OED can tell us, they (in their physical meanings) are not metaphorical extensions of some other word.
But if “tree” and “rock” aren’t metaphors, nearly everything else in our vocabulary seems to be.
For example, you cannot use “independent” without metaphor, unless you mean “not hanging from”. You can’t use “transpire” unless you mean “to breathe through”. The first English meaning of a book was “a written document”. If we want to avoid all metaphorised language (If we want to be “literal”), we must constantly rush to a historical dictionary and frantically check that there is no concrete meaning historically antecedent to the one we hope to use. In every language, pretty much everything is metaphor—even good old “literally”, the battle-axe of those who think that words can always be pinned down precisely.