Final answer:
Macbeth's dialogue in Act 3, Scene 1 emphasizes the theme that the quest for power is never fully satisfied, as he laments the witches' prophecy that only Banquo's lineage, not his own, will secure the throne.
Step-by-step explanation:
In Act 3, Scene 1 of Macbeth, Macbeth reflects on the witches' prophecy that Banquo's descendants will be kings, while his own lineage will not continue on the throne. Macbeth's commentary on the witches placing a fruitless crown on his head and a barren sceptre in his grip, only to be taken by an unlineal hand, enriches the universal theme of 'the quest for power' by suggesting that power and ambition can never be fully satisfied.
This dissatisfaction stems from the idea that Macbeth cannot secure his legacy and power for his own bloodline, which ultimately underscores the futility of unchecked ambition and the transitory nature of power. The mention of a barren sceptre and the gripe to let go of power to one who is not his descendant (unlineal) propels the theme that ambition is endless and can lead to one's downfall.
The reference to the fruitless crown ties into the continuous pursuit of power and the obsession with one's legacy, themes central to Shakespeare's work, and reflective of human nature's relentless pursuit of power, prominence, and security. Macbeth's torment over the idea that Banquo's children will become kings, while he himself has no such assurance, reveals both jealousy and the recognition of his own ambition's limitations.