What can we learn from analyzing Lady Macbeth's first soliloquy in Act 1, Scene 5 of Shakespeare's Macbeth in terms of main issue she is discussing and if the soliloquy is effective?
In Lady Macbeth's first soliloquy in Act 1, Scene 5, she is allowing her ambitious drive to get the better of her. She has just learned through Macbeth's letter to her that he was just made Dane of Cawdor and that the Three Witches had prophesied he would also be crowned king. Rather than being practical and considering dissuading him from doing anything rash for the mere sake of gaining power that is not rightfully his, she let's her own thirst for power ignite her own ambition, and she makes plans to encourage her husband to do whatever is needed to gain more power. This soliloquy is especially useful in portraying the dangers of excessive ambition, even portraying it as an illness.
Several literary devices are present that not only portray the dangers of excessive ambition but also portray Lady Macbeth as a particularly dangerous woman. One effective literary device portraying the dangers of both ambition and Lady Macbeth can be seen in her metaphor in which she compares her husband to a young baby still feeding on a mother's milk, milk she likens to "human kindness," as we see in her lines:
Yet do I fear thy nature;
It is too full o' the milk of human kindness
to catch the nearest way [to becoming crowned king]. (V.i.14-16)
In these lines, if Macbeth is "too full [of] milk," then he is being likened to an innocent infant, as infants are also full of milk since it's what they feed upon. In addition, she uses the phrase "milk of human kindness" to show just how much Macbeth has been nurtured by kindness, making him an equally kind person. If she objects to Macbeth's innocent kind nature, then we also know that this passage is painting Lady Macbeth as a particularly dangerous woman, someone to be feared and distrusted. It is also foreshadowing exactly how much she will soon negatively influence Macbeth's kind nature.
Another effective literary device can be seen in Lady Macbeth's use of antithesis in the lines, "[Thou] [a]rt not without ambition, but without / The illness should attend it" (17-18). Antithesis is a form of parallelism in which one grammatically balanced sentence expresses opposing ideas. Dr. Wheeler gives us the example, "Evil men fear authority; good men cherish it" ("Rhetorical Schemes"). Since "fear" and "cherish" are exact opposites, and since both clauses of the sentence are grammatically parallel, this is a perfect example of antithesis. Similarly, Lady Macbeth asserts that Macbeth is "not without ambition" and then makes the antithetical claim that he is "without the illness" that drives ambition, meaning that he does not have the obsession that will drive him to do anything at any extent to fulfill his ambitions. The parallel antithetical structure especially emphasizes the word "illness," which actually perfectly characterizes Lady Macbeth's own illness. Lady Macbeth is so obsessed with power and ambition that she'll stop at nothing. Hence, even this literary device of using parallel antithetical structure effectively portrays excessive ambition as an illness and Lady Macbeth as a very dangerous woman.