Simplify the following text for a Grade 9 native English-speaking person.
Original: The history of Caligula 's reign is extremely problematic as only two sources contemporary with Caligula have survived â '' the works of Philo and Seneca .
Simplified: The history of Caligula 's reign is a problem . Only two sources contemporary with Caligula have survived â '' the works of Philo and Seneca , and they give mostly anecdotes .