Why is it true that Poseidon is furious? Because his temple was desecrated. Here is a bit more, because I know the Poseidon example is a bit annoying:

• Poseidon being furious is a state of mind, one that cannot necessarily be posed as fact. His emotion is speculative as a statement alone, therefore the author is offering the argument to prove Poseidon’s emotion. The author is positing the Poseidon must be furious because his temple was desecrated.