The author of Holy Writ is God, in whose power it is to signify His meaning, not by words only (as man also can do), but also by things themselves. So, whereas in every other science things are signified by words, this science has the property, that the things signified by the words have themselves also a signification. Therefore that first signification whereby words signify things belongs to the first sense, the historical or literal. That signification whereby things signified by words have themselves also a signification is called the spiritual sense, which is based on the literal, and presupposes it. (ST Ia 1.10.c)
Words signify things by convention but how do things themselves signify other things? How does God make one thing a sign of another thing? The answer seems at first obvious: through similitude. This is how poets make allegory; they make whatever they describe similar to what they want it to signify. But the problem with this view is that “similar” is such a vague and elastic predicate—what seems similar to one interpreter does not to another. This has brought the spiritual sense into disrepute with many exegetes. One “historical-critical” scholar says of the spiritual interpreters of the Song of Songs that if they happen to give the same interpretation of anything it is only because they are copying each other. I once heard Jeremy Holmes suggest a solution to this difficulty. The similitude in question, he argued, comes from anticipatory participation, one thing signifies another by participating in it. David’s kingship, for example, would signify Christ’s because it participates in in it. Thus one cannot arbitrarily interpret anything which seems in anyway similar to another thing to be a sign of that thing, but one must show that the sign participates in what it signifies.
Strictly speaking, though, this could only work for two of the three spiritual senses:
Now this spiritual sense has a threefold division. For as the Apostle says (Hebrews 10:1) the Old Law is a figure of the New Law, and Dionysius says (Coel. Hier. i) "the New Law itself is a figure of future glory." Again, in the New Law, whatever our Head has done is a type of what we ought to do. Therefore, so far as the things of the Old Law signify the things of the New Law, there is the allegorical sense; so far as the things done in Christ, or so far as the things which signify Christ, are types of what we ought to do, there is the moral sense. But so far as they signify what relates to eternal glory, there is the anagogical sense. (Ibid.)
The moral sense obviously does not signify what we ought to do by participating in it—rather the other way around. And here one can see an application of Joseph Butler’s principle of The Analogy of Religion, Natural and Revealed, to the Constitution and Course of Nature: God’s work in revelation is analogous to his work in nature. How does one thing signify another in nature? Either as an effect signifies its cause (smoke is a sign of fire) or as cause signifies its effect (a North wind is a sign of the approach of bad weather). And thus in the allegorical and anagogical sense things signify the exemplar causes of which they are the effects while in the moral sense the exemplar signifies its effects.