I feel some tremendous downward energy from this first movement of 13th quartet by Beethoven.
He apparently has tenacity for 4 descending notes as I will describe in detail here. It seems to be a bit different from his usual attachments to his motives. I guess that his reason is deeply related to the Grand Fugue. As you may agree, Grand Fugue is filled with the continual ascending will and expanding energy. Therefore he has to make an introduction movement which has the clear opposite subjects for contrast. Unique construction of the first movement eloquently speaks that its final movement must be Grand Fugue.
First movement is composed by Sonata form but its structure is very unique. This movement is characterized by the first 4 notes of introduction.
Fig.1 shows and introduction part. The motive b’-a’-as’-g’ that descends with increasing volume is most important motive that competes with the theme of Grand Fugue and finally yields.