The business of the writer is to arouse in the mind of his reader the
fullest possible consciousness of the ideas or emotion that he is
expressing.

To this end he suggests a comparison between it and something else
which is similar to it in respect of those qualities to which he
desires to draw attention. The reader's mind at once gets to work
unconsciously on this comparison, rejecting the unlike qualities and
recognizing with an enhanced and satisfied consciousness the like
ones. The functions of simile and metaphor are the same in this
respect.

Both simile and metaphor are best when not too close to the idea they
express, that is, when they have not many qualities in common with it
which are not cogent to the aspect under consideration.

The test of a well-used metaphor is that it should completely fulfil
this function: there should be no by-products of imagery which
distract from the poet's aim, and vitiate and weaken the desired
consciousness.

A simile, in general, need not be so close as a metaphor, because the
point of resemblance is indicated, whereas in a metaphor this is left
to the reader to discover.

When a simile or metaphor is from the material to the immaterial, or
vice versa, the analogy should be more complete than when it is
between two things on the same plane: when they are on different
planes there is less dullness (that is, less failure to produce
consciousness), and the greater mental effort required of the reader