These two men are not alone; a third is present and listening, and it is he who completes the line left suspended by the messenger's words, "To add the death of you/ Merciful heaven". He urges the bereaved man to give sorrow words, to be comforted and to dispute it like a man with "us".
"Let's make us med'cines of our great revenge/ To cure this deadly grief." To which the man whose life of incurable grief is just beginning famously responds: "He has no children." We cannot tell for certain whom he means by "he" - whether the man who is trying to comfort him too promptly or the man who has killed his children. He might have said "Thou hast no children", or "You have no children". It is not the only occasion in Macbeth where it is not clear who "he" is.
Pronouns help us work out who we are, you are, they are, and their singular equivalents. In the theatre, pronouns acquire a radical urgency because they are wrought into the conditions of performance. They remind us at a less than fully conscious level that we are all performing these pronouns all the time, whether we like it or not. The three men in this scene do have names of their own: Ross, who brings the news; Macduff, who receives it; and Malcolm, who listens and intervenes. But in the theatre we do not hear these proper names as we hear the pronouns that enact the relations between them: I, you, thou, he.
I want now to set up some thoughts about "the third person". Let me swiftly sketch a spectrum of beliefs and practices. At a mundane level there is the legal position of the "third party', that is to say, "a party or person besides the two primarily concerned", as in the third-party insurance familiar to car-drivers.
At a more fabulous level, we may think of the tripled daughters and sisters of myth and folk-tale, of whom the third represents "that which shall be", or in Freud's tragic scenario, the Goddess of Death in masquerade as Cordelia, Aphrodite, Cinderella and Psyche. Less paganly, we may think of the Holy Ghost as the Third Person of the Trinity, or of Christ on the road to Emmaus, or of the figure in TS Eliot's "What the Thunder Said": "Who is the third who walks always beside you?" The figure of the third is always ominous, whether of good or of ill, of black magic or white.
We should also think of the superstitious ideas of third bodily organs. Apart from our fingers and toes, we mainly think of our basic corporeal endowment in terms of ones and twos. We normally greet with alarm the idea of two heads or three nostrils.
To have a third nipple was no joke in Shakespeare's time, but a matter of life or death for those suspected of witchcraft. Macbeth makes a nervous joke in response to the Second Apparition, the bloody child who cries: "Macbeth, Macbeth, Macbeth", to which Macbeth replies: "Had I three ears, I'd hear thee." But this play holds so many triple happenings and utterances that perhaps one does need a third ear.
Perhaps one could do with the "third eye" familiar to Hindu and Buddhist belief. We might also pause over the "third tongue" - the figurative sense, now obsolete, of "a backbiter, slanderer", a false witness. Or between a husband and wife, so that one might think of the character of Iago as exactly "the third tongue', who comes between Othello and Desdemona.
My own emphasis is on the ethical significance of this figure. The third person may stand at the edge of the scene, a bystander and looker-on, like so many attendant lords and servants. I am particularly interested in the moment when such a figure "comes forward" and steps into a scene between two (or more) others. Of course he or she or they may signally fail to do so; or they may be positively turned away and ejected, no longer one of us, to speak and be spoken to but only spoken about.
We do not go to tragedy for fantasies of immunity. Macbeth reminds us that there is no safe place for the third person, not even for the reader. We should attend to the predicament of those onlookers, witnesses and bystanders whose choices and fates prefigure our own, as we endlessly turn from him and her to thee and you and me and us, playing our parts and taking them, making and unmaking our one common world.