[Boromir88]

I agree with Form's post, when Gandalf tells Denethor he's "a steward" as well, it's more Tolkien making a commentary on the difference between Denethor's Stewardship and Gandalf's. There are different meanings to the word and Tolkien is contrasting the two meanings by using Gandalf and Denethor.

(all quotes from Minas Tirith)

"Yet the Lord of Gondor is not to be made the tool of other men's purposes, however worthy. And to him there is no purpose higher in the world as it now stands than the good of Gondor; and the rule of Gondor, my lord, is mind and no other man's, unless the king should come again."

This part is interesting because Denethor is flat out saying he rules Gondor, he is the "Lord," and thus the good of Gondor is his only purpose. Originally the house of Stewards, in Gondor, was designed to rule the realm when the King was gone.

When Earnur chases off after the Witch-King after being insulted, the Steward, Mardil stays and rules Gondor until Earnur is able to return. Although, in this case, Earnur doesn't return as he is killed, ending Anarion's line and Mardil becomes what would be the 1st "Ruling Steward." The Ruling Stewards all took oaths to hold the throne and sceptre until a king returns. Overtime the Stewards took these oaths out of tradition, and even though legally they could never claim the title of king (as it was decided only the line of Anarion could rule Gondor and the House of Stewards were not from Anarion's line), they essentially ruled as kings and weren't interested in giving up their power. As is evidenced when Denethor said Aragorn's house was "long bereft of Lordship," and his "unless the king should come again" comment is far from convincing. Something that Gandalf notices.

"Unless the king should come again?" said Gandalf. "Well, my lord Steward, it is your task to keep some kingdom still against that event, which few now look to see. In that task you shall have all the aid that you are pleased to ask for. But I will say this: the rule of no realm is mine, neither of Gondor nor any other, great or small. But all worthy things that are in peril as the world now stands, those are my care. And for my part, I shall not wholly fail of my task, though Gondor should perish, if anything passes through this night that can still grow fair and bear fruit and flower again in the days to come. For I also am a steward. Did you not know?"

There's a lot of things going on here, but look at the capitilization when Gandalf addresses Denethor as "lord Steward" and then when Gandalf says he's a "steward." The former he associates with a position of rule. Denethor says Gondor is his rule. The latter "steward" Gandalf associates with care. All things are Gandalf's care, in fact he rules nothing.

Gandalf is pointing to the humbler origins of stewardship, as opposed to Denethor's idea of Stewardship. The humbler origins have a religious context of stewards being like shephards to their flock. Denethor's idea of Stewardship is asserting his authority in Gondor.

If I've read Form's post correctly, I guess I'm saying Gandalf is actually doing the exact opposite. He is using the humble origins of what it is to be a "steward" to actually say he's in no legal, or authoritative position. He rules nothing, but cares for all. And goes to contrast Denethor's legal position of Gondor, as a "Steward."