Aristotle Rhetoric 3.4.4

1. As the shield is to Ares, so is the goblet to Dionysus. Proportion is defined ( Aristot. Nic. Eth. 5.3.8 ) as an equality of ratios, implying four terms at the least, and the proportional metaphor is one in which the second term is to the first as the fourth is to the third; for then one can by metaphor substitute the fourth for the second, or the second for the fourth. Let A be Dionysus, B a goblet, C Ares, D a shield. Then by the definition, the goblet is to Dionysus as the shield is to Ares. The metaphor consists in transferring to the goblet the name belonging to its analogue the shield. Sometimes an addition is made by way of explanation of the word in its new sense, and the goblet may be described as the shield of Dionysus and the shield as the goblet of Ares. The shield and the goblet both come under the same genus, being characteristics of a deity, and can therefore be reciprocally transferred ( Aristot. Poet. 21.4 ).