IV. When we have analysed the document and determined the literal meaning of its phrases, we cannot even yet be sure that we have reached the real thoughts of the author. It is possible that he may have used some expressions in an oblique sense; there are several kinds of cases where this occurs: allegory and symbolism, jests and hoaxes, allusion and implication, even the ordinary figures of speech, metaphor, hyperbole, litotes.[140] In all these cases it is necessary to pierce through the literal meaning to the real meaning, which the author has purposely disguised under an inexact form. Logically the problem is very embarrassing: there is no fixed external criterion by which we can make sure of detecting an oblique sense; in the case of the hoax, which in the present century has become a branch of literature, it is an essential part of the author's plan to leave no indication which would betray the jest. In practice we may be morally certain that an author is not using an oblique sense wherever his prime object is to be understood; we are therefore not likely to meet with difficulties of this kind in official documents, in charters, and in historical narratives. In all these cases the general form of the document permits us to assume that it is written in the literal sense of the words. On the other hand, we must be prepared for oblique senses when the author had other interests than that of being understood, or when he wrote for a public which could understand his allusions and read between the lines, or when his readers, in virtue of a religious or literary initiation, might be expected to understand his symbolisms and figures of speech. This is the case with religious texts, private letters, and all those literary works which form so large a part of the documents on antiquity. Thus the art of recognising and determining hidden meanings in texts has always occupied a large space in the theory of _hermeneutic_[141] (which is Greek for interpretative criticism), and in the _exegesis_ of the sacred texts and of classical authors. The different modes of introducing an oblique sense behind the literal sense are too varied, and depend too much on special circumstances, for it to be possible to reduce the art of detecting them to definite rules. Only one general principle can be laid down, and that is, that when the literal sense is absurd, incoherent, or obscure, or in contradiction with the ideas of the author or the facts known to him, then we ought to presume an oblique sense.