Saruman, just before he imprisons Gandalf, says the following:
The Elder Days are gone. The Middle Days are passing. The Younger Days are beginning. The time of the Elves is over, but our time is at hand: the world of Men, which we must rule.
Saruman and Gandalf are Maiar and they are speaking alone so why would he say this?
The answers to this question say that what we are reading is the story as told by Hobbits who didn't know the true nature of Istari. Howerver, this particular passage seems to come from Gandalf's precise account of his conversation with Saruman. It seems that Saruman is trying to make a very specific argument to persuade Gandalf to his plan which doesn't seem to make much sense since neither of them is a human.
I'm not necessarily looking for an in-universe explanation. So I am also interested in knowing whether at the time of writing these words Tolkien has already decided the precise nature of the Wizards.