In Act 5.1, Where is the doctor 'horrified at the truth he has learnt'?
Act 5 is where the doctor appears. He has been called by Lady Macbeth's gentlewoman because, as the gentlewoman explains to the doctor in scene 1 of that act, Lady Macbeth has been walking and talking in her sleep and she insists always on having lighted candles around her to dispel the darkness. While he is there, the doctor is witness to Lady Macbeth's nighttime wanderings and mutterings. He hears her say, "Yet who would have thought the old man to have had so much blood in him?" This was obviously a reference to Duncan and the doctor tells the gentlewoman that there are rumors going around about Macbeth's possible involvement in Duncan's murder. Lady Macbeth also mentions Macduff's wife, who has recently been killed along with her family. The things that Lady Macbeth says implicate her and her husband in much treachery and violence. The doctor does not want to hear what Lady Macbeth is saying because then he would know the truth and might possibly be swept up in the dirt along with the Macbeths.