Chavenet's one line critique of The Murders on the Rue Morgue was misunderstood by his contemporaries, not surprisingly, as Chavenet did not make public his views on plot and coincidence. They remain unpublished, hidden away in his notebooks, scattered among his notes on the 19th century masters of plotting.
The test of the technical skill of any writer is in the way they handle coincidences. A plot may be seen as a series of knots. In order to unravel the knot, a device rather like that of the sword of Alexander is needed: a coincidence. We can distinguish two types of coincidence.
The first type, which we may provisionally call intrinsic, happens at the level of the story only. In this type, the coincidence, the sword to cut the knot, is provided by something already existing in the discourse. The coincidence appears as such only to the characters on the level of the plot, but not to the reader on the level of the discourse. To the reader, the coincidence appears as design, a pattern in which the repetition of a detail previously skated over by the reader reveals previously unnoticed significations. The two appearances of the detail: the earlier foreshadowing the later, the later echoing the earlier, knits the work together and creates a satisfying and beautiful sense of unity. This type of coincidence is far superior. It shows careful plotting, artistic skill, and creates a revelation of meaning in the mind of the reader.
Dickens - at least in his later works- was a master of this kind of coincidence. In Bleak House, for example, the clue to the relationship between Lady Dedlock and Esther Summerson is provided by the letters found earlier in the novel in the possession of the deceased Nemo. The sword to cut the knot has already been provided by the discourse, and in the words of Barthes, acts as a seed which will grow later in the novel.
The second type, which we call here extrinsic, operates at the level of the story and discourse. Both character and reader perceive the coincidence as coincidence. In this type of coincidence, the sword to cut the knot is not previously provided by the discourse, but enters the discourse for the first time only at the precise moment when it is needed by the plot. This type of coincidence - literally far-fetched - fetched from far outside the discourse only when needed- is inherently inferior, and shows a lack of skill in plotting. The reader feels cheated by the arbitrary inclusion of a detail he was not able to foresee, a detail not already intrinsic to the discourse, and whose only purpose is to extricate the plotter from the mess of the plot.
It's this type of coincidence which features in Poe's story. Having taken great care to tie the plot up into a knot in which all rational explanations are ruled out - that is, in which all the details already provided by the discourse cannot explain or unravel the knot- at that moment, a sword to cut the knot is provided by the introduction of an Orang Outan, a being that has not appeared in the discourse until now. The story abounds in this kind of inferior, extrinsic coincidence. The explanation for the presence of the Orang Outan is provided by the story only at the moment when it is needed. Likewise, the red ribbon, the special knot, and Dupin's knowledge of the origin of the knot, are only provided at the moment when they are needed; they come from outside the discourse as it stands up until that moment, and operate as a deus ex machina, not for the characters, so much as for the poor author who has got himself into such a pother with his plotting.
Chavenet always discounted the possibility that the introduction of an Orang Outan - instead of some other creature, such as a burglar or other human actant -was deliberate on the part of Poe. He dismissed the notion that the inclusion of a creature so far removed from the world of the story and what Barthes calls the climate of the discourse - was actually a conscious attempt to foreground these two types of coincidence by ironically making the device which unlocks the plot from outside an utterly ludicrous, alien element. He attributed it instead to incompetence in plotting.
What seems more interesting, he maintained, was that the two types of coincidence represented a change in literary consciousness regarding the attitude towards free will and predestination. In the use of intrinsic coincidence, the author is revealed as being in control, the seemingly arbitrary details are revealed later as having significance, everything has a purpose in the overall design: an analogy of a worldview that posits a Great Author and a Design. The extrinsic coincidence, on the other hand, is more analogous to a world view which emphasises the arbitrary and unguided, the absurd, and the sheer preposterous patternlessness of reality.