The "sound argument" of the title refers to the cold logic that sacrificing one individual (Historia) is a necessary evil to save the nation.
If you are referring to the episode " A Sound Argument " (Season 4, Episode 10), it is a story centered on the heavy burden of duty and the erosion of hope. 10 : A Sound Argument
: In a poignant flashback, Eren Jaeger vehemently refuses to let Historia be used as a "breeding tool". The "sound argument" of the title refers to
: In the present day, we see the tragic outcome. Historia is shown pregnant and despondent, seemingly having accepted a fate she never wanted. : In the present day, we see the tragic outcome
The story begins with the arrival of Lady Kiyomi Azumabito, the first foreign guest to visit the walls of Paradis. She reveals a hidden lineage: is the lost descendant of the Shogun of Hizuru. While the news brings a glimmer of a possible ally, it comes with a steep price tag: Zeke Yeager’s "Three-Point Plan" for Eldia’s survival. Zeke's Plan and the Sacrifice Zeke’s strategy involves:
: Paradis must spend 50 years modernizing their military to compete with the world.