Book I, Chap. 5 of Finnegans Wake rewrites (reinterprets) the married life of HCE and ALP. It also explores the main subjects of the work such as language, family, and sexuality again in a particular way. The letter, called as a "mamafesta," a mother`s feast, a writing of a woman (ALP), is central in the chapter since it is expected to provide the key to uncover the truth of the "sin" of HCE. The letter unearthed by a hen is presented for the examination by a parody figure of pedantry, the Shaun-type narrator. He introduces the apparatus of scholarship such as textual, historical, Freudian, and Marxist analyses to explore the real meaning of the letter. He attempts to apply various theories and approaches to the interpretation of the letter, which can be regarded as an analogy of the Finnegans Wake. The focus of the Book I, Chap. 5 of Finnegans Wake is on the letter, its arrangement of words and the deciphering of its meaning, but the major subject is about the reading and understanding Finnegans Wake. In it we can find Joyce`s own defense and explanation of his embarrassing text. In the process of the examination of the letter`s authorship, content, and origin, the narrator/Joyce discusses the textual mechanism of the letter/Finnegans Wake. As the words in the letter is "variously inflected, differently pronounced, otherwise spelled, changeably meaning vocable scriptsigns," their meanings in it are continually "moving and changing every part of the time" (118.22-28), and they are even further reduced to the alphabet to show the numerous examples of the unstable relationship between the signifier and the signified. Although there is a deep relationship between the letter and Finnegans Wake, they cannot be identified unequivocally; however, in this essay I propose to demonstrate how their textual natures are identical in some perspectives.