One critic has said that the witches supply Macbeth with the nouns and Lady Macbeth supplies him with the verb. That is, the witches plant the seeds of "Thane of Cawdor," "Thane of Glamis," and "King Hereafter." This sparks Macbeth to write his wife of the news. By the time he gets home, she supplies him with the plan of "murder."
I like Amy's analogy of the witches "planting the seeds." But, the seed of blind ambition was always there. After all, it is his tragic flaw. The witches and Lady Macbeth give Macbeth the rationale to water the seeds, to cultivate this blind ambition.