Full context:   Modal logic is an extension of classical logic. In its original form, sometimes called "alethic modal logic", it introduces two new symbols: ◊ expresses that something is possible while ◻ expresses that something is necessary. For example, if the formula B ( s ) stands for the sentence "Socrates is a banker" then the formula ◊ B ( s ) articulates the sentence "It is possible that Socrates is a banker". To include these symbols in the logical formalism, modal logic introduces new rules of inference that govern what role they play in inferences. One rule of inference states that, if something is necessary, then it is also possible. This means that ◊ A follows from ◻ A . Another principle states that if a proposition is necessary then its negation is impossible and vice versa. This means that ◻ A is equivalent to ¬ ◊ ¬ A .