How did Denethor know about Aragorn?

[CAB]

I agree and disagree with you here, Alcuin.

Of course you are right that Denethor was already aware of Aragorn (aka: Thorongil, the leader of Pippin and Boromir's company after Moria, Heir of Isildur). What was interesting about The Gaffer's suggestion wasn't the thought that Denethor may learned about Aragorn from Sauron. Rather, it was the idea that Sauron (who was almost surely unaware of Denethor's knowledge concerning Aragorn) may have intended to use this information to further weaken Denethor, and through him, Gondor.

"... let folly be our cloak, a veil before the eyes of the Enemy! For he is very wise, and weighs all things to a nicety in the scales of his malice. But the only measure that he knows is desire, desire for power; and so he judges all hearts."

As I argued earlier in this thread, the one thing Sauron probably understood best about Denethor was his desire to retain his power. Aragorn's imminent return would not have given Denethor hope, it would have done just the opposite. And, to a degree, Sauron was correct in his thinking regarding Denethor.

"I would have things as they were in all the days of my life," answered Denethor, "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."

Originally Posted by Alcuin

Since Sauron believed either Aragorn or Gandalf had the Ring, he knew that Denethor did not. So he must have been bluffing Denethor in order to weaken his will to fight before Aragorn could arrive.

I don't think bluffing was necessary in this case. According to Sauron's way of thinking, Denethor, once made aware of Aragorn, should have lost his willingness to fight. He was in, as The Gaffer said earlier, a lose-lose situation.