It is difficult to know anything for certain about Caligula. Only two sources from his time still exist. Those are the works of Philo and Seneca. They give mostly anecdotes. At one time, there were detailed histories on Caligula. Now they are lost. Caligula's sister, Agrippina the Younger, wrote an autobiography which certainly had a lot of information about Caligula's reign. It is also lost. No surviving source paints Caligula in a favorable light. It is not known whether Caligula was insane.