Paraphrasing the Summa: First part, Question 1, Article 10

Question 1: The nature and extent of sacred doctrine.

Is Christian doctrine revealed by God?  Does it teach us about the supernatural as well as the natural?

Article 10. Whether in Holy Scripture a word may have several senses?

Is Sacred Scripture literal in all which it teaches?  Or does it express itself in other senses of the words used?

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.

If a word can have more than one meaning, then the result is confusion rather than effective communication.  Therefore, Sacred Scripture can't have more than one sense in 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.

Even in this series of objections, there is no agreement as to the various senses of Scripture.  Therefore, it does not seem wise that Scripture should be divided into senses of Scripture with which even its adherents can not agree.

Objection 3. Further, besides these senses, there is the parabolical, which is not one of these four.

And, there is even an other sense which has not been mentioned.  The parabolical (i.e. parable). 

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."

St. Gregory the Great, in his book "Moralia", chapter 20, p 1, says that Sacred Scripture must convey more than one sense because in every sentence, it explains the word of God.

I answer that, The author of Holy Writ is God,

The primary author of Scripture 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.

God's greatness is such that words do not suffice to convey His meaning as men are thus limited.

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.

But Christian doctrine conveys meaning by the express meaning of the words and by things which the words can indirectly signify.

Therefore that first signification whereby words signify things belongs to the first sense, the historical or literal.

Therefore, the first meaning is the 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.

Then, there is a spiritual meaning which is based upon the literal and must not contradict the literal. 

Now this spiritual sense has a threefold division.

The spiritual sense can be divided into three categories. 

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.

Because, it is known that the New Law was foreshadowed in the Old.  And the New is itself a figure of that which is to come.  And in the New Law, all which Christ did is an example of that which we should do.  So the three sense of the spiritual sense are as follows. 

Therefore, so far as the things of the Old Law signify the things of the New Law, there is the allegorical sense;

The allegorical sense.  Because the Old Law is an allegory of the New. 

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.

The moral sense.  Because all which Christ did for us, is what we should also do. 

But so far as they signify what relates to eternal glory, there is the anagogical sense.

The anagogical sense.  Because all which we do in Christ leads to our justification, sanctification and ultimate salvation.

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.

Since Holy Scripture is written by a  human author inspired by the Spirit of God, then it is presupposed that the words written will have more than one sense.  

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.

There is no confusion by the many senses of Scripture.  Because all the senses are based upon and do not contradict the 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.

Although the various Saints focus on certain senses of Scripture and list what appear to be different categories. They do not, in fact, contradict, when they are examined minutely. For instance, in St. Augustines example, he merely divided the literal sense into three categories. And instead of listing the three spiritual senses, he reduced them to one, allegory. Thus, Hugh of St. Victor doesn't list four but three senses, collapsing the anagogical and allegorical together.

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.

The literal sense can be expressed by the use of figures.  For example, if we speak of God's arm, we don't mean that God has an actual limb.  But that God has power to do the thing being discussed.  So, although the literal sense belongs to the human author, the human author was also not limited to the literal sense of the word for his expressing of God's truths.