How did Sauron not know about the Ring?

Question

It says in The Fellowship of the Ring that Sauron learned that the One Ring was still in existence, partially from Gollum. Gandalf says, during his explanation to Frodo,

He believed that the One had perished; that the Elves had destroyed it... ¹. 

However, how could he have thought this if he was reincarnated? I thought that after Isildur defeated Sauron he was reduced "back into shadow" and that the only reason he was able to reappear was because the Ring still existed. So wouldn't Sauron have known the Ring was still around, because otherwise he couldn't have came back?

―――
^{1 Fellowship of the Ring, Book I Chapter 2: "The Shadow of the Past", two pages after Gandalf throws the ring into the fire.}

Answer 1

The Ring, and the portion of Sauron's power within it, is indeed what tied him to Middle-Earth, but there is no reason he had to be aware of this. If Sauron had information from a source he believed reliable that the One Ring had been destroyed, he would be likely to believe that his continued existence was due to keeping a greater part of his power in himself than he had previously thought.

Answer 2

It says in the fellowship of the ring that Sauron learned that the One Ring was still in existence, partially from Gollum.

No, this is not correct.

He learnt from Gollum that he once had a ring –

After the creature Gollum, who had previously possessed the ring, was captured, Sauron had him tortured and learned that he once had a magic ring, and from him he heard the words Shire and Baggins. He deduced that Gollum's ring was the One Ring, and sent his servants the Nine to find Shire and search for Baggins, so that the One Ring might be found and brought back to him.

Source

He deduced that Gollum's ring was the One Ring.

That's entirely different to learning that the One Ring was still in existence.

If the One Ring had been destroyed then Sauron would have been destroyed with it. The fact that Sauron wasn't destroyed means that the One Ring was still in existence (and Sauron knew it still existed) but didn't know where it was or who possessed it.

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One Ring Timeline

Second Age 1500, Celebrimbor forges the 16 rings under the instruction of Annatar, who is Sauron in disguise.

Second Age 1600, Sauron forges the One Ring to rule all the others.

Second Age 1700, Gil-galad and Tar-Minastrir destroy Saurons army forcing him back to Mordor to regroup.

Second Age 3261, Ar-Pharazôn, the last of the Kings of Númenor, do battle with Sauron in contention of his self-proclaimed title of Overlord of Middle-Earth. Sauron is defeated and captured but through the use of the One Ring is able to turn the Númenóreans against Valar and towards worship of Melkor and human sacrifice.

Second Age 3429, Although Sauron's body is destroyed in the Fall of Númenor, his spirt was able to bear the One Ring back to middle earth and he wielded it in his renewed war against the Last Alliance of Elves and Men. The One Ring is cut from Saurons hand by Isilidur.

Third Age 2, Isildur loses the ring in the River Anduin just before he is killed by an orc ambush.

It's at this point that Gandalf suggests in The Fellowship of the Ring that Sauron thinks the One Ring is destroyed. Notice there's almost 2,400 years between the One Ring being lost and Déagol finding it in the River Gladden.

Third Age 2430, Déagol finds the ring in the River Gladden, a tributary of the Anduin and is killed by Sméagol for it.

Notice that Sméagol/Gollum possesses the One Ring for almost 500 years before Bilbo finds it. Sauron is fully aware of its existence during this time.

Third Age 2941, Bilbo finds the One Ring.

Notice how Bilbo only has the One Ring for about 60 years.

Third Age 3001, Bilbo hands the One Ring to Frodo.

Third Age 3018, Sméagol dies in Mount Doom and the One Ring is destroyed.

Answer 3

There is a bit of a contradiction here.

In 2941 of the Third Age, Bilbo finds the Ring. However, in the article "On the Rings of Power and the Third Age" he wrote (more or less)

In T.A. 2851, after a meeting of the White Council, Saruman began searching near the Gladden Fields for the One Ring.[3] He became alarmed to discover that Sauron's servants were also searching the region.[9]

So if Sauron thought the Ring was destroyed at the start of the Age, what was he doing searching Gladden Fields decades before Bilbo takes it from Gollum?

Answer 4

From the back-and-forth discussions and references quoted in answers above, it seems logical to conclude (as logical as we can make a fictional person acting within fictional events):

1. Sauron was defeated and the ring cut from his hand. His spirit was diminished to the point of non-existence, or at least near-absolute non-consciousness for quite some time. He was oblivious to the actions (or inaction) of Isildur and the Ring.

2. Sauron's spirit slowly starts to re-materialize within Middle Earth. Likely wandering aimlessly, although still filled with his usual malice and memories of what had been done to him. At this early point in his resurgence He thinks his Precious ring was destroyed at the same time he was, likely by Isildur or the Elves (Gandalf's interpretation anyway).

3. Time passes and he continues to grow in strength. His Nazgul lieutenants are still active (the Witch King of Angmar in particular), and start wreaking general havoc within the fractured kingdoms of Men and Elves of the time. Here we could infer that Sauron may have begun to suspect that his Ring may not have been destroyed, but merely lost.

4. Sauron ends up in Mirkwood under his guise as the Necromancer. I suspect by this point he knows about the One Ring's continued existence and that it's not far from Mirkwood (which lies next to both the Gladden Fields and Misty Mountains, making his choice of a home base seem pretty reasonable). He probably learns through various means of Isildur's ultimate fate and how the ring 'fell out of all knowledge' around this time. He starts sending scouts and agents afield to search for it, just as Saruman has started doing.

5. Years later, after reestablishing himself within Mordor, he captures Gollum and finally confirms the existence of his One Ring and that it was held for a time by this miserable creature. He now knows it must be in the possession of a 'Baggins' in a place called 'Shire', and sets things in motion to retrieve it to restore him to full power and assert his mastery over Middle Earth.

So it's not inconceivable that at one point early in Sauron's 'disembodied spirit' days he truly believed his ring was destroyed, and had no idea that it was it's continued existence that kept his Spirit bound within Middle Earth (and allowed it to grow in strength over time). It may have been merely a temporary gap in his understanding of how intertwined both his fate and that of the Ring's truly was.

But it's also likely he soon surmised the nature and reason of his continued existence, and took actions to try and find that which was lost. His questioning of Gollum merely confirmed what he already long suspected, and filled in a crucial 'missing piece of the puzzle' of what happened to the ring after it was taken from him.