Demonic inversions ( such as the line fair is foul and foul is fair ) are frequently seen in the play . Another possible reference made in Macbeth was to equivocation , as Henry Garnettâ s A Treatise of Equivocation was found on one of the plotters , and a resultant fear was that Jesuits could evade the truth through equivocation : Inversions seen in such lines as `` fair is foul and foul is fair '' are used frequently , and another possible reference to the plot relates to the use of equivocation ; Garnettâ s A Treatise of Equivocation was found on one of the plotters .