Live and dead metaphors
In all discussion of m. it must be borne in mind that some metaphors are living, i.e. are offered and accepted with a consciousness of their nature as substitutes for their literal eqivalents, while others are dead, i.e. have been so often used that speaker and hearer have
ceased to be aware that the words used are not literal. But the line of distinction between the live and the dead is a shifting one, the dead being sometimes liable, under the stimulus of an affinity or a repulsion, to galvanic stirrings indistinguishable from life. Thus, in
The men were sifting meal we have a literal use of sift; in Satan hath desired to have you, that he may sift you as wheat, sift is a live m.; in the sifting of evidence, the m. is so familiar that it is about equal chances whether sifting or
examination will be used, and that a sieve is not present to the thought--unless indeed someone conjures it up by saying All the evidence must first be sifted with acid tests, or with the microscope. Under such a stimulus our m. turns out to have been not dead but dormant. The other word, examine, will do well enough as an example of the real stone-dead m.; the Latin examino, being from examen the tongue of a balance, meant originally to weigh; but, through weighing is not done with acid tests or microscopes any more than sifting, examine gives no convulsive twitches, like sift, at finding itself in their company. Examine, then, is dead m., and sift only half dead, or three-quarters.
A Dictionary of Modern English Usage