Since this is the last movie in the 6-part saga, Ep.3 needs to give us an explanation of the prophecy . Personally I think that vader & sidious think that vader has fulfilled the prophecy of the chosen one already by eliminating the jedi. In ROTJ Vader tells Luke that the emperor will show him the "true" nature of the force, so the sith might believe that blocking out certain feelings (like anger, hate, aggression) is wrong and that you should, as sidious says, "learn to trust your feelings" and that their use of the force is the right way since it's more natural to have these kinds of feelings. The reason I thought about this subject is because I was reading another thread and someone said that sidious might not believe that anakin is the chosen one, well I dont think that has to be the case at all, sidious might really think that the chosen one (vader) has brought "balance" to the force and this is probably what he tells vader in ep.3 ROTS, because if the prophecy is not really specific then it's open for different types of force users to have their own interpretations of the prophecy, just we like have different branches of Christianity all based around Christ but slightly different beliefs or ways of worship, the sith and jedi might all have their beliefs based around the Prophecy but each believes that their use of the force is the right one. This is why this NEEDS to be explained in this last movie. What do you guys think?