According to Monty Python: King Arthur and his Knights, having grenaded a vicious rabbit with pointy teeth, enter the cave of Caerbannog to find the location of the Holy Grail carved on the wall:
ARTHUR: What does it say?
MAYNARD: It reads, ‘Here may be found the last words of Joseph of Arimathea. He who is valiant and pure of spirit may find the Holy Grail in the Castle of uuggggggh’.
ARTHUR: What?
MAYNARD: ‘The Castle of uuggggggh’.
BEDEVERE: What is that?
MAYNARD: He must have died while carving it.
LAUNCELOT: Oh, come on!
Because he allegedly died while writing it, Johann Sebastian Bach’s Art of Fugue can generate mythology which, while dramatic, possibly cinematic, and full of otherworldly coincidences, may not be so constructive (see also: Dan Brown and the Holy Grail). But at least the sources of the mystery in Bach’s case are clear. First, the existing autograph manuscript of Contrapunctus XIV (unfinished) contains a note from his son Carl Philip Emanuel: ‘At the point where the composer introduces the name BACH in the countersubject to this fugue, the composer died.’ It is impossible to judge the son’s intentions (which, however truly respectful, may have been of a marketing sort), but it appears at least that the son’s account does not accurately correspond to events. By the time of his death, Bach’s handwriting had become uncertain in a way that this manuscript, written in Bach’s hand, does not demonstrate. So it seems most likely that, at the moment he wrote the manuscript, the elder Bach was ‘not dead’ (see Monty Python again) and quite possibly feeling fine.
The second source for reverential obfuscation is that the Art of Fugue is such an unbelievably masterful composition; there is a Pythagorean complexity which can seem cryptic and impossibly multilayered. However, as an introduction to the mechanisms and techniques of the work (mechanisms which are also hard at work in Schubert’s Eb trio), we can at least give a short demonstration of the workings of two canons 
that introduce today’s concert. The canon alla Duodecima is a relatively straightforward one, in which the decorated main theme (A, bare; B, decorated) is copied at an interval of a twelfth, with an eight-bar delay. This process is then repeated with the roles traded and intervals inverted. The canons per augmentationem (in contrario motu) use the same theme (A), but decorated differently (C) 
and used very differently: this time, the second entrance is not transposed by interval, but is instead flipped upside down and doubled in length (D).
These are merely a few canons – not enough music to explain or explore the Art of Fugue, really, but certainly enough to demonstrate techniques of invention and aural/mnemonic challenge which composers have been using for centuries, and which they will continue to use in one form or another for centuries to come, no matter the medium. This is simply one of the ways that music works.