Why do you think critics are so sure that someone other than Shakespeare's wrote Hecate's speech?

Answer 1

There are a number of reasons. Hecate and the witches talk differently in the disputed scenes (Act 3 Scene 5 and parts of Act 4 Scene 1) from the way they do elsewhere (in 1,1 and 1,3). In Act 1 their speech is terse and mysterious. But Hecate talks like Titania from A Midsummer Night's Dream ("Upon the corner of the moon There hangs a vaporous drop profound; I'll catch it ere it come to ground" or "And now about the cauldron sing Like elves and fairies in a ring.") She is cutesy and silly, not threatening.

There are references to two songs which the witches are to sing in the disputed scenes-- "Come away, come away", and "Black Spirits"--accompanied no doubt with a song-and-dance number. These songs are not by Shakespeare; they are from Thomas Middleton's play The Witch. And the speeches for Hecate sound like the characters in that play. Look it up if you like; the text still exists and is available online.

And in addition, Hecate's plan revealed in her Act 3 Scene 5 speech is to control Macbeth by making him overconfident with an overconfidence potion, so he'll come a cropper. But such a development sucks all the interest out of the play: we are interested in what will happen to Macbeth and Lady Macbeth because of the guilt they feel over Duncan's murder. If Macbeth's behaviour is due to magical intervention none of that matters.

So the reasons add up to this--this sounds like Middleton, not Shakespeare, and some of it is indisputably Middleton. It is inconsistent with the portrayal of the witches elsewhere and the structure of the play as a whole. And let's be honest: what director would want to have a song in the middle of the cauldron scene with the scintillating line "Titty, Tiffin, keep it stiff in."? The answer is, of course, none.