The Power of the Witches

I wonder whether the three witches truly have the power to speak the future or whether they are simply nudging MacBeth towards an inevitable fate.  Banquo says that devils often tell half truths in order to "win us to our harm."  I believe this is the case for MacBeth.  Their foretelling moved him to the point of killing Duncan.  While Banquo wisely lets events take their natural course, it is MacBeth that is taken over by this curse.  Throughout the play, the three witches serve as the unconscious temptations of the human mind.  Their dark, supernatural aura makes the characters believe their foresights, but I believe that their power comes through understanding the vulnerabilities of each individual and preying on their weaknesses.  They tell the future of MacBeth by telling him what he subconsciously wants to hear.  The witches prey on his ambition and serve as a subconscious driving force behind his actions.