(5.3) mostly develops the themes of fearlessness, nihilism, and psychological afflictions. MacBeth takes it literally that "no man that's born of woman / shall e'er have power upon" him-and forgets that women can deliver by Cesarean.
MacBeth is sick at heart, and feels he has "liv'd long enough" so he tends to approach everything now from a nihilist's perspective. This is true especially in his "tomorrow, and tomorrow, and tomorrow" soliloquy.
Finally, the theme of psychological affliction appears as MacBeth tells the doctor that he cannot "cure her of that" and cannot "minister to a mind diseased" or "pluck from the memory a rooted sorrow" or "raze out the written troubles of the brain."