St. Réal's novel was published in 1672 and has been a favorite quarry of the dramatist. Of the plays of Otway (1676) and Campistron (1685) Schiller had no knowledge, nor did he receive any suggestions from the fierce and gloomy 'Filippo' of Alfieri, which appeared in 1783. He approached the subject in his own way and his first thought was simply to dramatize St. Réal, who is mainly interested in the love tragedy and writes as a literary artist rather than as a political or religious pamphleteer. We possess a prose outline[65] of 'Don Carlos', written probably at Bauerbach, which shows exactly how the theme first bit into Schiller's mind. The exposition was to show the secret passion of the lovers and the dangers threatening them from the jealousy of Philip, the political hostility of the grandees and the malice of the slighted Eboli. In the third act the king would become madly suspicious and resolve upon his son's death. Then there was to be a gleam of hope: the ambition of Carlos would awaken and begin to prevail over his love, while Posa would divert the king's suspicion to himself and fall a sacrifice to friendship. Then a new danger would arise: the king would discover Don Carlos in a seeming 'rebellion', and decree his death. The dying declaration of Carlos would prove his innocence and the king would be left alone to mourn the havoc he had wrought and to punish the conspirators who had deceived him.
This sketch promises, it will be observed, not a political tragedy, but, as Schiller himself afterwards phrased it, a 'domestic tragedy in a royal household'. Springing up from the same soil and at the same time as 'Cabal and Love', it was to be much the same sort of play. In both a pair of high-minded lovers belonging together by natural affinity, but separated by artificial barriers; the rights of passion battling in the one case with social prejudice, in the other with the law of Rome and the malice of courtiers; in both a court plot against the lovers; the hero beset by a fair sinner who receives him in her private room, lays siege to him, and is angered by the slighting of her love; in both a tyrannical and headstrong father at enmity with his son. Of the political ideas which the world associates with 'Don Carlos' there is here no adumbration. We hear nothing of the Netherlanders, nor of the Inquisition, nor of the rights of man. Posa is only a friend of Carlos, not the ambassador of all mankind, and there is no room for his golden dreams of philanthropic statesmanship. And yet it is worth noticing that in three points (all in the third act) Schiller adds to his French source: Carlos's ambition was to waken and prevail over his love, Posa was to sacrifice himself, and the lovers were to rise superior to their passion.