For two almost entirely opposite reasons, both having to do with Finnegans Wake, no list of allusions in Joyce's work can claim to be definitive. In the first place, the Wake requires a more unrestrained search than would be conducted in a less obscure and less highly wrought text. At the risk of lengthening his list with false identifications, the compiler must consider many allusions that can only be called “possible.” The second difficulty comes from the opposing need to define some limits to a search in the Wake. Glasheen puts an extreme case in her Third Census: “Every ‘is’ indicates Issy and it is out of [the] question to list them all.” And this problem is compounded by the book's principle of correspondences – “everybody is somebody else.” For example, if Issy “is” on some level Isolde, then is every “Issy” an allusion to Tristan und Isolde? Is every “is”? To follow out the implications of this principle in creating this appendix would trivialize the enterprise, producing a list of ridiculous length and dubious value.

Here the method of selection is to judge each case on its own merits – a reference to “Venus,” for example, may be listed as an allusion to Tannhäuser if its context seems to draw on themes from the opera or if another reference to Wagner is present – but to err on the side of inclusion.