Sauron and the palantir

[Angry Hill Troll]

In the various writings about the Palantiri in the UT, in some instances there is a mention of an eighth Master Stone In Tol Eressëa, but in others there isn't. It's not quite clear what JRRT's final decision was on this issue.

It may be that Sauron wasn't able to completely control the Stone to gather military intelligence precisely because he wasn't a legitimate user. Aragorn was (the attack of the Corsairs) and so was Denethor--also a legitimate user as the heir of the Steward to the last King of Gondor. In UT, Tolkien discusses in some detail the differences between Saruman and Denethor. Both got into trouble by using the Palantiri, but Saruman (an illegitimate user who uses the Palantir for selfish motives) ends up (more or less) doing Sauron's bidding. Denethor, whose motive is to fulfill his duty to Gondor, continues to oppose Sauron, athough he is tricked into believing the situation is hopeless--a much less catastrophic fall than Saruman's.

Of course if it had been desirable for Gandalf to look into the Palantir, all he would have had to do to be a legitimate user and so gain an advantage over Sauron was to ask Aragorn's permission.