Objection 1. It seems that in Holy Writ a word cannot have several senses, historical or literal, allegorical, tropological or moral, and anagogical. For many different senses in one text produce confusion and deception and destroy all force of argument. Hence no argument, but only fallacies, can be deduced from a multiplicity of propositions. But Holy Writ ought to be able to state the truth without any fallacy. Therefore in it there cannot be several senses to a word.
Objection 2. Further, Augustine says (De util. cred. iii) that "the Old Testament has a fourfold division as to history, etiology, analogy and allegory." Now these four seem altogether different from the four divisions mentioned in the first objection. Therefore it does not seem fitting to explain the same word of Holy Writ according to the four different senses mentioned above.
Objection 3. Further, besides these senses, there is the parabolical, which is not one of these four.
On the contrary, Gregory says (Moral. xx, 1): "Holy Writ by the manner of its speech transcends every science, because in one and the same sentence, while it describes a fact, it reveals a mystery."
I answer that, 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. 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. Since the literal sense is that which the author intends, and since the author of Holy Writ is God, Who by one act comprehends all things by His intellect, it is not unfitting, as Augustine says (Confess. xii), if, even according to the literal sense, one word in Holy Writ should have several senses.
Reply to Objection 1. The multiplicity of these senses does not produce equivocation or any other kind of multiplicity, seeing that these senses are not multiplied because one word signifies several things, but because the things signified by the words can be themselves types of other things. Thus in Holy Writ no confusion results, for all the senses are founded on one — the literal — from which alone can any argument be drawn, and not from those intended in allegory, as Augustine says (Epis. 48). Nevertheless, nothing of Holy Scripture perishes on account of this, since nothing necessary to faith is contained under the spiritual sense which is not elsewhere put forward by the Scripture in its literal sense.
Reply to Objection 2. These three — history, etiology, analogy — are grouped under the literal sense. For it is called history, as Augustine expounds (Epis. 48), whenever anything is simply related; it is called etiology when its cause is assigned, as when Our Lord gave the reason why Moses allowed the putting away of wives — namely, on account of the hardness of men's hearts; it is called analogy whenever the truth of one text of Scripture is shown not to contradict the truth of another. Of these four, allegory alone stands for the three spiritual senses. Thus Hugh of St. Victor (Sacram. iv, 4 Prolog.) includes the anagogical under the allegorical sense, laying down three senses only — the historical, the allegorical, and the tropological.
Reply to Objection 3. The parabolical sense is contained in the literal, for by words things are signified properly and figuratively. Nor is the figure itself, but that which is figured, the literal sense. When Scripture speaks of God's arm, the literal sense is not that God has such a member, but only what is signified by this member, namely operative power. Hence it is plain that nothing false can ever underlie the literal sense of Holy Writ.
Source: Summa Theologica (New Advent)