There are many strange things said about St. Thomas’ teaching on analogous terms. Karl Barth claimed that the only reason not to be a Catholic was analogy, and the constant repetition of the phrase “the analogy of being” makes it seem as though being is analogous in the same way that it is true or good, or perhaps that to be is to be analogous.
Regardless of whether these claims are true, they are not the first thing that is true about analogy. Analogy is in fact so familiar and frequent that most of the time we don’t bother to notice it. An analogous term is a single sound with many related meanings. Since almost all of the terms we use have many related meanings, we don’t tend to give them a distinct name like “analogous”. It is simply taken for granted that a word is analogous.
Again, I stress that words as a rule are analogous, and in fact the more a word is used, the more analogous it becomes. Every common noun in English is usually a verb also: put “to” in from of a noun and most English speakers will know what you mean. Adjectives can usually become nouns if one just put a “the” in front of them. Prepositions are notoriously analogous: try to figure out how any language uses its “to” or “of” or “in” or “at”. As soon as one thinks of a verb in the infinitive, he can usually turn it into a noun. And so on forever. All of these changes, which are made effortlessly and often without thought, make drastic differences in the meaning of what we are saying. For example, we English speakers easily jump from nouns to verbs, but it is a very different thing to exist per se (the mode in which nouns signify) and to exist in another and as dependent (the mode in which verbs and adjectives signify).
Although any term can give an example of analogous naming, St. Thomas usually uses a single example borrowed from the practice of medicine: animals are healthy, medicine is healthy, etc. St. Thomas had two very good reasons to use this example: first, it would have been well known to his audience that it came from Book IV of the Metaphysics, and so St. Thomas could call to mind a whole book in a single example; and secondly, St. Thomas could tie what he was saying explicitly to the teaching of Aristotle, which was one of his primary goals in all of his thought and writing. It is not, however, an ideal example to make clear what analogy is as such, because Aristotle does not offer that example to illustrate analogy as such, but to illustrate how analogous terms can form a single science. The best example of the generation of analogous terms as such comes from the relation of cause and effect, which is exactly the very ground for why we can speak about God existing at all.
To explain: a man hammers, and a hammer hammers. A single action, therefore, is said of both the agent and the instrument. Yet the action is radically and even essentially different in both: for men, the action is a vital action, for a hammer it is not; for a man, it is an action being done by himself and not though another, for a hammer it is not; for a man it is an action coming forth from within and from his knowledge, and for a hammer it is not; for a man, “to hammer” means “to use a hammer” and for a hammer it does not. We can go on listing differences like this for quite some time and yet for all these differences it does not become one wit less rational to refer to the activity of the agent and instrument by the same word. This points to one of the central things that people fail to understand about analogy: a single term is often used not in order to express a single meaning, but a single operation, or some other kind of unity in the things themselves. It would not be any perfection of language, as some think, to have a single term or symbol for every distinct meaning. Such an imaginary language would be gravely imperfect and philosophically worthless, in spite of having some accidental perfections. Words are subordinated to our understanding of reality, and to have one term for every meaning would distort things and mislead us in understanding reality. Note also that not only would it be undesirable to use two words for an agent and the instrument, it might simply be impossible. Say we call a man hammering “firsthammering” and leave the action of the instrument the same. Still, “firsthammering” and “hammering” are a single action that both perform. Now, of course, we have to distinguish between two senses of perform, note the unity of them, distinguish that unity into “first___” and another, and so ad infinitum.
The idea that being or existence (or any positive term) is said analogously of God and creatures is a direct result of the very way we prove God exists at all. As St. Thomas explains, we prove God exists because we posit something that obviously exists (motion, or contingency), and then show how it is being caused by something “all call God”. What this means is that we are showing how something relates to God as an instrument to an agent, or if you like as a second caused cause relates to the primary cause. We say both God and creatures exist for the much the same reason we say men and hammers hammer. This radical distinction in existence will lead, a fortiori to a radical distinction of any other positive trait of existence: justice, mercy, love, strength, knowledge, etc.