Sauron
I don't think he was thinking foolishly. Put yourself in his place. We have heard the argument that he only thinks in terms of greed and malice. But also think practically.
Frodo and
Sam going into
Mordor to destroy
the ring is about as practical as a naked man walking into Fort Knox to make an unauthorized withdrawal. Who would think it? It would surely seem the stupidest thing to do.
Someone said why should he fear
Aragorn? Well, Aragorn is the descendant of the guy who defeated him and stole his ring. The
sword he wields struck him down. And through the
Palantir Sauron and Aragorn "wrestled in thought" and Aragorn wrested control of the stone from Sauron. So he has a strong will. Strong enough to wield the ring.
Nowhere I've seen does it say Sauron is invincible while the ring exists. Both
Gandalf and
Galadriel have aluded to the fact that they COULD defeat Sauron with the ring at their command. It just needs a powerful enough being to bend the ring to his will. In the end they would be corrupted and become LIKE Sauron, but Sauron himself would be gone. So yes, Sauron has reason to fear. And were does this mere mortal get the cheek to present himself before Sauron in the first place? He MUST have the ring to be so self-assured.
Think of it this way. The ring is a trigger for a bunch of nukes. The good guys have it, but using it will destroy their lands as well. If Sauron gets it, he is ASSURED victory. In the meantime, he has overwhelming conventional forces so he can probably win even without it. So in that position, and not having a love for the land that everyone else does, he would definitely think Aragorn has put on the ring and is coming to kick his butt out of Mordor.
Eol