Final answer:
The rhetorical device used in the underlined excerpt from Plato's The Republic is antithesis, contrasting the outer appearance with the inner self to highlight the significance of internal justice.
Step-by-step explanation:
In the excerpt from Plato's The Republic, the rhetorical device used in the underlined portion is antithesis. The underlined text contrasts the outer appearance of a person with their inner self, highlighting the distinction between the superficial and the substantial components of human character. Antithesis emphasizes this difference by juxtaposing opposing ideas within the same sentence or argument, serving to enhance the meaning and provide a clearer understanding of the concept of justice as internal harmony and self-governance.