According to St. Thomas, God is the author of the spiritual sense and the literal sense. In point of fact, St. Thomas suggests that the literal sense may signify more even than the human author comprehends, since even the literal sense relies primarily on that signification which God intends (ST I, q.1, a.10). Needless to say, the spiritual sense, it seems, always pertains exclusively to the divine intention and must always go beyond the comprehension of the human author whose powers are unable to signifying things with other things.
Yet, it may be questioned whether, in the case of a metaphor, the thing literally signified is the metaphorical object, the thing figured, or both. St. Thomas contends that the figure (the metaphorical object) is not that which is signified, but rather the thing figured is alone signified according to the literal sense. It must be remembered that metaphor is contained in the literal sense of the text—poetry and metaphor use words to signify things under figures, but this is still far short of signifying a thing with another thing.
St. Thomas’ fundamental teaching regarding the spiritual sense is that it is always founded upon the literal and proceeds from it (Quaestiones de quolibet VII, q.6, a.1). A second theme regarding the spiritual sense is that the truth signified by the spiritual sense in one place - insofar as that truth is necessary to the faith - is signified according to the literal sense in some other place (ST I, q.1, a.10). As the spiritual sense is always founded on the literal sense, so too any essential truth signified by the spiritual sense is also signified according to the literal sense of some other passage.
Most importantly, St. Thomas’ method of literal interpretation is marked by his careful study of each word of the text—each word is studied in itself, in relation to the rest of Scripture, and in relation to truth as presented more generally through science, philosophy, and theology. It is this focus on the meaning of each word which characterizes the literal interpretation of Scripture - since the literal sense is that by which the words themselves signify things.
In contrast to St. Thomas’ position, modern scholarship places the literal sense entirely in the intention of the human author. Thus, the literal sense is no longer directly associated with the manner in which words communicate truth, but rather with the intention of the human author of the text. Thus the Pontifical Biblical Commission in The Interpretation of the Bible in the Church defines the literal sense as follows: “... the precise meaning of the texts as produced by their authors...” (IBC, II, B. 1) Referring to “authors” in the plural, it is clear that the Commission intends the human authors and not the divine author - who is, in fact, the primary author. Moreover, this is made explicit when the Commission again defines the literal sense as “that which has been expressed directly by the inspired human author.” In what appears to be an after-thought, the Commission adds that “this sense is also intended by God.”
It is obvious that the Thomistic understanding is very different from the modern approach. For St. Thomas, the literal sense refers to the meaning of the words, which meaning is intended by God as the primary author. For modern Scripture studies, the literal sense is that meaning which the human author intends. Thus, much of what St. Thomas would classify as the literal sense (for example, many of the prophecies of the Old Testament are considered to literally refer to Christ), modern biblical scholarship would consider the “spiritual sense”. It seems difficult to reconcile this modern approach with the teaching of the fathers and doctors of the Church.