Background to the concept of analogy key to understand St Thomas Aquinas:Analogy stands between equivocality and univocality:
A univocal term has only one possible meaning (no place for confusion, so to speak), eg "entomology".
An equivocal term can mean very different things, for instance "bark" which without any background can equally mean "the outer covering of a tree" or "the short, loud sound made by dogs".
An analogical term is similar (partially the same, partially different) to another.
There were three types of analogy used in the Middle Ages:
1. Analogy of proportionality: for instance "principle" is an analogical term when said of a point and a spring of water because a point is related to a line as a spring is related to a river.
2. Analogy of attribution: eg analogy involved a relation between two things, of which one is primary and the other secondary.
3. Analogy of participation: the most important example here is the analogia entis, the way in which God, an angel, a man, a zebra, a birch and a sapphire "are," though each containing different degrees of being and, in the case of God, being Himself the principle of being, the Ipsum Esse Subsistens.
We could add a fourth one mentioned during our discussion, "improper" analogy or metaphor, ie an analogy in which there is no real basis for similarity. For instance when we say someone is "on fire" we don't mean (hopefully) that flames are consuming that person's clothes, but that he is working or doing something with enthusiasm and energy. Or we can say someone is "a gem" but clearly we don't mean she is a piece of stone, but someone who has lend a very valuable service.
You can read a very nice summary of all these notions by clicking here.
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