IF deeper pitfalls are laid by anything more than by the facts of coincidence, it is perhaps by the intimations and suggestions of writings which bear, or are held to bear, on their surface the seals of allegory and, still more, of dual allusion; as in the cases of coincidence, so in these, it is necessary for the historical student to stand zealously on his guard and not to acknowledge second meaning or claims implied, however plausible, unless they are controlled and strengthened by independent evidence. Even with this precaution, his work will remain anxious, for the lineal path is difficult to find and follow. Perhaps there is one consolation offered by the gentle life of letters. In matters of interpretation, if always to succeed is denied us, to have deserved it is at least something. Among our aids there is one aid which arises from the correspondences between distinct systems of allegory and symbolism. They are important within their own sphere; and it is by subsidiary lights of this nature that research can be directed occasionally into new tracks, from which unexpected and perhaps indubitable results may be derived ultimately. When the existence of a secondary and concealed meaning seems therefore inferentially certain in a given department of literature--if ordinary processes, depending on evidence of the external kind, have been found wanting--its purpose and intention may be ascertained by a comparison with other secret literatures, which is equivalent to saying that the firmest hermeneutical ground in such cases must be sought in evidence which inheres and is common to several department of cryptic writing. It is in this way that the prepared mind moves through the world of criticism as through outward worlds of discovery. I am about to set forth after a new manner, and chiefly for the use of English mystics, the nature of the mystery which is enshrined in the old romance-literature of the Holy Graal. As a literature it can be approached from several standpoints; and at the root it has a direct consanguinity with other mysteries, belonging to the more secret life of the soul.