Merry, referring back to your earlier comment below:
Denethor is certainly 'empirial' in his person and his rule and, because of that, in both situations, no son or son-substitute would ever have been good enough. I can't remember if this is in the book or the movie or both, but somebody says to Denethor that if Boromir had taken the Ring, Denethor would not have recognized his son.
Denethor most certainly felt that Gandalf was attempting to usurp him on several fronts. First, by usurping the affections of his youngest son Faramir, turning him into a "Wizard's pupil." But Denethor's most revealing statement is the following quote from "The Pyre of Denethor":
"Do I not know thee, Mithrandir? Thy hope is to rule in my stead, to stand behind every throne, north, south, or west. I have read thy mind and its policies . . . . With the left hand thou wouldst use me for a little while as a shield against Mordor, and with the right bring up this Ranger of the North to supplant me.
"But I say to thee, Gandalf Mithrandir, I will not be thy tool! I am Steward of the House of Anarion. I will not step down to be the dotard chamberlain of an upstart. Even were his claim proved to me, still he comes but of the line of Isildur. I will not bow to such a one, last of a ragged house long bereft of lordship and dignity . . . .
"I would have things as they were in all the days of my life . . . and in the days of my longfathers before me: to be the Lord of this City in peace, and leave my chair to a son after me, who would be his own master and no wizard's pupil. But if doom denies this to me, then I will have naught: neither life diminished, nor love halved, nor honour abated."
PJ used some of this in the throneroom scene between Gandalf, Pippin, and Denethor in ROTK. Really strong stuff. Denethor's greatest flaw was pride, which Sauron certainly twisted to his advantage through the palantir. In the Appendices, we learn that Denethor either strongly suspected or outright knew who Thorongil was:
But there was little love between Denethor and Gandalf; and after the days of Ecthelion there was less welcome for the Grey Pilgrim in Minas Tirith. Therefore later, when all was made clear, many believed that Denethor, who was subtle in mind and looked further and deeper than other men of his day, had discovered who this stranger Thorongil in truth was, and suspected that he and Mithrandir designed to supplant him.
It must have roiled Denethor almost beyond reason to be given glimpses through the palantir of Aragorn in company with Boromir and the Fellowship being led by Mithrandir. All of his suspicions were coming true!
And you are right Merry. It was Gandalf himself who told Denethor the following about Boromir and the Ring in "The Siege of Gondor":
"In no case would Boromir have brought it to you. He is dead, and died well; may he sleep in peace! Yet you deceive yourself. He would have stretched out his hand to this thing, and taking it he would have fallen. He would have kept it for his own, and when he returned you would not have known your son."