Thou Shalt Not Mix Thy Metaphors

Thou shalt see what thou writest; and therefore thou shalt not mix thy metaphors. For a mixed metaphor is proof that the image therein contained has not been seen with the inner eye, and therefore such a metaphor is not a true metaphor, created by the active eye of imagination, but stale jargon idly drawn up from the stagnant sump of commonplace.

HUGH TREVOR-ROPER