It is the soul of a philosopher who died of starvation contemplating the universe.

Eidolon of Rhetoric

Each player can't cast more than one spell each turn.

Theros's human philosophers were at a marked disadvantage due to their need to pause for things like food or sleep.

Jumpstart 2022

The List

Journey into Nyx

Eidolon of Rhetoric will look at the entire turn to see if a player has cast a spell yet that turn, even if Eidolon of Rhetoric wasn't on the battlefield when that spell was cast. Specifically, you can't cast Eidolon of Rhetoric and then cast another spell that turn.