Charles Rossman, ‘The Reader’s Role in A Portrait of the Artist as a Young Man’, in James Joyce: An International Perspective, ed. Suheil Bushrui & Bernard Benstock (Gerrards Cross: Colin Smythe 1982), pp.19ff.
Joyce’s much discussed idea of the ‘epiphany’ is basically a theory of perception. Stephen Dedalus makes this clear when, near the end of Stephen Hero, he expounds the theory, in words too familiar to quote at length. A few excerpted phrases will make my point: ‘By an epiphany he meant a sudden spiritual manifestation. … He told Cranly that the Ballast Office clock was capable of an epiphany. … All at once I see it and I know what it is: epiphany. … Imagine my glimpses at that clock as the gropings of a spiritual eye which seeks to adjust its vision to an exact focus. The moment the focus is reached the object is epiphanised’ (SH, p.211).’ Stephen states the matter explicitly. The thing (the Ballast Office clock) is out there; the perceiver needs to adjust his focus carefully; under ideal conditions, an epiphany occurs: object and observer coincide to produce a pellucid ‘reality’.
‘In an earlier, less familiar passage, Stephen corroborates this notion of perception as a delicate attunement of perceiver to object. He speaks of the artist’s special faculty of perception as the ability to ‘disentangle the subtle soul of the image from its mesh of defining circumstances’ (SH, p. 78). In A Portrait, Stephen Dedalus advances a similar notion of perception. He explains to Lynch how, confronted by a basket, the ‘mind first of all separates the basket from the rest of the visible universe which is not a basket’, in order to ‘apprehend it as one thing’. This phase of perception Stephen calls the discovery of the object’s integritas. Two successive phases, the discovery of consonantia and of claritas, yield a radiant manifestation of ‘the whatness of a thing’. That is, the mind discovers ‘that thing which [the basket] is and no other thing’ (AP, p.213).
‘Joyce and Stephen agree, at least, on their theories of perception. Joyce’s own youthful essays, notebooks, and letters frequently echo Stephen’s belief that the mind can apprehend the objective world, that the observer can enter into and interpret phenomena. In “James Clarence Mangan”, for instance, Joyce scolds those of “impatient temper” who disregard “any method which bends upon these present things and so works upon them and fashions them that the quick intelligence may go beyond them to their meaning, which is still unuttered.” Although Joyce here has in mind the way that an artist transmutes experience into art, his remark is also an early formulation of both Stephen’s description of the “artist’s selective faculty”of “disentangling the subtle soul of the image” and Stephen’s more elaborately theoretical notions of the epiphany and of integritas, consonantia, and claritas.
What is most pertinent to this discussion is that Joyce’s words in ‘James Clarence Mangan’ also describe the reader’s experience of Dubliners, A Portrait, and much of Ulysses . Joyce has arranged the facts in these books so that the reader is led toward complex, epiphanic, narrative moments which reveal a situation, a circumstance, or a character but leave their “meaning … still unuttered”. The reader must engage these books by attending closely to the literary facts, ‘these present things,’ in order to go beyond them to their meanings. In particular, the reader’s task is to discover the potential meanings of the epiphanic moments. Wolgang Iser has shown how Ulysses creates “aesthetic blanks” that the reader fills, how Joyce prestructures potential meanings that are discovered and actualized through the process of reading [ The Implied Reader, John Hopkins UP 1974, xii, 179-233]. What Iser has demonstrated about the design of Ulysses holds true for Joyce’s earlier  books as well. They are also prestructured with “blanks” or “gaps” that the reader fills by discovering potential meanings. It is especially instructive for readers of A Portrait, seeking clues to that book, to examine how Joyce creates such ‘aesthetic blanks’ in Dubliners .’ (pp.25-26.)
‘From the green rose on the opening page of the novel to the “loveliness which has not yet come into the world’ on one of the last pages, from a child’s claiming “his song” to a young artist crystallizing a statement of his artistic goals - such is the allotropic sequence that will frame, for the competent reader,  Stephen’s final and grandest statement about flight to implement an imagined ideal: “Welcome, O life!” […’, &c., AP, Viking Press 1968, pp.252-23; here pp.32-33.)
I have stressed filling aesthetic blanks and adducing contexts because these entail a wide range of readerly activities. Two such activities demand further attention: getting the literary facts straight, and assessing perspectives properly, both those of the characters who offer and interpret facts and that of the narrator, whose selection and ordering of facts, and whose tone, finally lead us to the author’s predetermined interpretations. The example of Michael Furey, Gretta, and Gabriel illustrates the elusiveness of fact and the ambiguity of meaning. In A Portrait even more than in “The Dead”, we are teased by elusive facts and ambiguous meanings. For instance, a reader cannot be certain why E. C. leaves without speaking to Stephen following the Whitsuntide play - or whether, for that matter, E.C. really attended the play in the first place. More crucially, readers continue to debate whether Stephen has a “wet  dream” prior to composing his vilanelle - a matter of fact which may determine our evaluation of the villanelle and, consequently, our interpretation of Stephen’s prospects at the book’s end. (p.33-34.)