The souls of plants and animals (or even the human soul so far as it possesses both in its singularity) do not survive death and so are compared to accidental forms, like roundness compares to a basketball. For that matter, all forms are known through accidental ones. But if this is the only comparison we use to understand the substantial form, we will be unable to get a clear view of its nature. The souls of plants and animals do not survive for a reason that is more like how carpentry could not survive in a world without tools. It is because of the loss of an ontologically inferior and subordinate principle that the whole is lost (even though this inferior principle is essential), not because what is more foundational or real is lost. This second comparison (some would say analogy) has its precise application too, and it “breaks down” just as the other one does (though this talk of analogies breaking down is really just a way of saying that they can be taken in ways they were not meant to be taken, since no one analogy suffices to explain the thing that is its analogue)
Metaphysics is like poetry in the sense that it advances by multiplying comparisons to things in the sensible world. By its very nature, no analogy is sufficiently proportionate or adequate to the thing that is its analogue (if it were, we would simply know it directly, without comparing it to another.) And so the very unified vision of a single metaphysical object requires the co-ordination and ordering of various diverse comparisons, and so when we strive to understand what is named by a single analogous term, like “form” or “life” or “being” we must use far more than one comparison. The unity of the term and its corresponding concept is not such that it is adequately explicated or even understood by a single comparison.
Notice that I have continually used the term “comparison” where the term “analogy” tends to get used. The two should not be conflated here. Both involve knowing a like thing through another that is also unlike it, but analogy speaks to the unity of the concept whereas what I here call the comparison is what needs to be multiplied to understand the single analogue. Said another way, so far as the analogy is manifestive of the other, it is unified, so far as it is inadequate to the other (so far as the analogy fails), it demands other comparisons to manifest the thing named analogously.
Just as it is helpful to understand the difference between a comparison and an analogy, it is also crucial to understand how both are distinguished from metaphor. The multiplication of comparisons looks a lot like poetry, but poetry turns on metaphor, which is a far more narrow category than comparison. Metaphors are improper comparisons, and do not extend the proper senses of what they are said of. When Macbeth calls life a “brief candle” we don’t include a new entry in the lexicon under “candle”. This is not a point about lexicons, but about the difference between a comparison extending a term to a second imposition (analogy) and it not doing so. The comparisons that the metaphysician uses really do terminate in new senses of words that are essentially secondary and cannot be understood by us except through another.