Does this appear to be a big interpretive jump on my part? Let's review E. T. A. Hoffmann's account. Remember his characterization of this theme: "genial serenity, confident awareness of its own strength and substance." We typically read and accept such descriptions without any real qualms--or else ignore them as mere platitude or sentimentalizing. Isn't it interesting how Hoffmann's words take on fresh significance given a more exhaustive semiotic interpretation? Hoffmann was hearing sensitively and musically (despite his incomplete formal analysis), but he simply had no theory to account for the more poetic side of his interpretation. Nevertheless, the literary Hoffmann often emphasized the poetic and the romantic aspects of music. In his short story "Ritter Gluck" (1972 : 8) he refers to the tonic and dominant (scale degrees) as giant colossi and the Tierce (third scale degree) as a soft youth with a sweet voice, an interesting confirmation of my own charaterization of start open fifth versus sweet third in my discussion of unusual tonic triad doublings in Beethoven (1994: 50-56).
There is another reason to argue for agency that goes beyond a theme's "self-awareness," and in this case, to argue for a single agency in these opening bars. It has to do with performance, and with the gestural interpretations to which performers must commit themselves. Listen to the opening 8 bars, perhaps comparing more than one performance.
In one performance there is a noticeable break, due to bowing, between m. 6 and 7 in the cello. The tension of gestural continuity is broken, if only momentarily, and we hear an interpretation in which the F-natural dies away, to be displaced by the F-sharp. But if the move from F-natural to F-sharp is crucially motivic, and it is part of a larger dramatic scheme as outlined above, then we must hear instead a kind of transformation, that F-natural as it were melts into F-sharp, without losing the gestural tension that, if properly executed, will carry us over the suspended time of bars 5 and 6.
That F to F# is indeed motivic may be argued from more evidence than its clear use here as part of a structural juncture, or expressive crux, that marks the beginning of the lyric Y motive with such unforgettable magic. The end of the exposition has a curious overlapping of harmonies that also blurs the final cadence in A, such that we focus more on the plagal close of IV to I. The potential perfect authentic cadence in mm. 62-63 is clouded by the move from a Vm9 to V7/IV, and the conflict between F-natural in the former (also viio7) vs. the F-sharp in the IV (anticipated by the trill) involves a similar transformation from tension (the diminished-seventh, dominant-ninth sonorities) to extremely relaxed consonance (the subdominant). While F to F# is pursued twice in the piano trills, and is echoed by G to G# to A for the elaborated plagal cadence, the cello has an equally telling move from G# to G-natural, undercutting the perfect authentic cadence in m. 62-63. This yielding reversal, in which a resignational pulling-down is more than compensated by the positive arrival on the subdominant, results in the trope of abnegation, or spiritual acceptance, that gives such appropriate expressive significance to this exquisitely subtle closural strategy. The piano trills allow an otherwise syntactically impossible concatenation of F-F# and G#-G to occur right at the point of negated syntactic closure, m. 63, and provide another expressive crux that sums up the goal of this expressive genre. When the tritone C#-G of violin and cello resolve in m. 64 to the subdominant, that effect is still further enhanced, and prolonged, by the analogous contrary motion by half-step. The expressive fulfillment is in the strings' re-engagement of a more melodic gesture derived from y, in m. 66, to close in the proper key of A major. Finally, the sublime character of extreme registers on the piano adds to the benediction of this substitutional plagal cadence, which has taken an expressive role analogous to the Picardy-third cadence in a minor-mode work.