At the start of Act I, scene vii, Macbeth is considering the technical parameters of a hypothetical murder, finding "If it were done, when ti's done, then 'twere well/It were done quickly" (ll.1-2). When his fears about the consequences of detection surface, Macbeth begins to list the reasons for not assassinating the king. He turns first to customary personal loyalty, observing that Duncan is a blood relative and, as such, that Macbeth should protect the king against knife rather than wield it against him. Secondarily, he says that Duncan has been a good king, against whom he has no grievance. But he fails to mention the most obvious reason for refraining from murder, that it is morally wrong, a cardinal sin that deserves damnation whether detected by human agency or not. Instead, he urns to making an inventory of the resources he would need, should he decide to move forward. On this count, Macbeth finds one thing lacking, "I have no spur/To prick the sides of my intent, but only/Vaulting ambition, which o'erleaps itself" (I, vii, l.25-27).
It is then that the "spur" appears (almost as if Macbeth had conjured it into being) as Lady Macbeth enters the scene. Initially, Macbeth is adamant in his rejection of the course that they both know must be taken if he is to become Scotland's ruler, and tells his wife that they shall proceed no further in this business. Now Lady Macbeth launches into her argument, and the "spur" that has captured the critic's attention is her charge that Macbeth is a coward. In fact, she does not directly say this (she merely asks if her is prepared to live like a coward), nor is it the crux of her case. Indeed, Macbeth has a rebuttal to the coward charge, asserting to Lady Macbeth, "I dare do all that may become a man;/Who dares do more is none" (I, vii, ll.46-47), and he then commands her to be silent. But Lady Macbeth need not heed her duty toward her husband, for there is a second plank to her counter-argument; she tells Macbeth that if he does not follow through on their developing plot then he has broken a promise to her. She first asks whether the "hope" that he raised for their royalty in the letter that he sent to her after meeting the witches was "drunk." She then says that since the expectations he raised in this missive were false, she will accord his professions of love to her to be equally false. That being so, some "beast" must have egged Macbeth on to breaking the bonds of trust with his wife by making promises (the attainment of the throne) that are then withdrawn.