Definition: Perfect equanimity.

References:

1. Epicurus believed that anyone could attain and maintain ataraxia.

Definition: A puzzle about identity through time.

References:

1. The puzzle is based on the story of Athenians who made a sacred journey to Delos on Theseus' ship each year. The ship required ongoing repairs, and over time the rotten planks were replaced with new ones, until all had been replaced. Plutarch describes Athenian philosophers who debated whether the ship was still the same after all the repairs.
2. Hobbes: if someone collected all the old planks of the ship and restoring the old ship is this also the Ship of Theseus? Does the same ship exist twice?