This paper proposes first, to examine the theoretical background of the ‘epiphany,’ James Joyce’s basic aesthetics of fiction, by relating his earlier writings to Thomas Aquinas’s aesthetics. Perusing Joyce’s biographical facts and diverse critical debates, I trace the etymological origin of ‘epiphany’ and then, how Joyce, transforming the originally religious term into a literary signifier, employs it as a basic narrative tool for his work. Second, by comparing Stephen Hero’s Chapter XXV and A Portrait’s Chapter V, I trace how Joyce’s selective adaptation of the former into the latter transforms his theory of ‘epiphany.’ Stephen in Stephen Hero, defining ‘epiphany’ as ‘a sudden spiritual revelation’ or ‘a memorable phase of the mind itself’ shown by ‘the vulgarity of speech or of gesture,’ positively embraces Thomas Aquinas’s position and tries to understand the recognition of beauty as a psychological experience shared between subject and object. Grasping human epistemology as a mutual penetration of the subject and object, he transfers his focus from the object’s three ontological elements of beauty, i.e. the object’s formal properties such as ‘integritas, consontia, claritas,’ to the subject’s psychology. The first stage of the recognition of beauty is to allow the object ‘integrity’ by drawing a demarcation between the object and its background. Such an allowance of independent ‘otherness’ to the object is the preliminary task for the subject to realize the object. In other words, the object’s autonomy is temporarily realized in the subject’s consciousness. The second stage starts to analyze the object’s formal properties by spotting the relationship between the whole and its parts, thus hitting upon the object’s inner structural logic. When the first and second stages are completed, the subject and object reach a moment of aesthetic unity, a radiant moment of transcendental oneness, when the object’s ‘whatness’ is revealed. Stephen calls this blissful moment ‘epiphany.’ Thorough research, however, betrays Stephen’s theory of epiphany as too hasty and incomplete. In conclusion, Stephen, a young artist in the making, makes an abrupt logical jump in his assertion, “Claritas is quidditas.” Stephen’s ‘applied Aquinas’ proves incorrect and incomplete, which ironically renders him to be an artist ‘as a young man.’