How does cicero interpret the signs casca reports to him?

As natural phenomena.

In Act 1, Scene 3, a very upset Casca bumps into Cicero on the street. He tells them all the strange phenomena he has witnessed: heaven dropping fire, a man with his hand ablaze but not burning, a lion in the Capitol and an owl at noon. Casca believes this is a sign that the Gods are fighting a war or plotting the destruction of Rome. Cicero, on the other hand, tells Casca to not rush to conclusions, as men often see what they want to see. Unconcerned, Cicero leaves, at the same time a Cassius enters. Cassius, however, upon hearing of the events, is worried, as he interprets them as a warning for Caesar.

nature versus humanity!