It may be [he wrote] that in the first [three] acts I have aroused expectations which the last do not fulfill. St. Réal's novel, perhaps also my own remarks upon it in the first number of the _Thalia_, may have suggested to the reader a standpoint from which the work can no longer be regarded. During the period of elaboration, which on account of divers interruptions was a pretty long time, much changed within myself.... What had mainly attracted me at first, attracted me less later on, and at last hardly at all. New ideas that came into my mind crowded out the earlier ones. Carlos himself had declined in my favor, for no other reason perhaps than that I had outgrown him, and for the opposite reason the Marquis of Posa had taken his place. So it came about that I brought a very different heart to the fourth and fifth acts. Yet the first three were already in the hands of the public, and the plan of the whole could not be recast; I had either to suppress the piece entirely (for which very few of my readers would have thanked me), or else to fit the second half to the first as best I could.
Let us look somewhat closely at the process of evolution here alluded to in general terms.
The original impulse came from a work of romantic fiction, the 'Dom Carlos' of St. Réal, which was first read by Schiller in the summer of 1782 and drew from him the comment that the story 'deserved the brush of a dramatist'. St. Réal's novel begins by telling how Charles the Fifth arranged, just before his abdication, that his grandson Carlos should some day marry Elizabeth of Valois: and how afterwards Philip determined to take the French princess for his own wife instead of leaving her to his son. Meanwhile, however, by much gazing at the picture of his betrothed, young Carlos had learned to love her, and she in turn had conceived for him a 'disposition to love rather than a veritable passion'. Arrived at the Spanish court the young queen wins all hearts; even the white-haired Philip falls in love with her, though he treats her with stately reserve in the presence of others and surrounds her with the restraints of Spanish etiquette. Thus the queen comes to feel that she possesses 'only the body of her husband, his soul being filled with the designs of his ambition and the meditation of his policy'. As for Carlos, his love-lorn eyes soon betray to her how it is with him, but she can only pity him, though she secretly returns his love, for she is as virtuous as she is beautiful.