He thinks that the customs of his tribe and island are the laws of nature

From Caesar and Cleopatra, Act II by George Bernard Shaw.

Britannus (shocked):
Caesar, this is not proper.

Theodotus (outraged):
How?

Caesar (recovering his self-possession):
Pardon him Theodotus: he is a barbarian, and thinks that the customs of his tribe and island are the laws of nature.