The predicative genitive
This use of the genitive is characterised by an impersonal use of est followed by a genitive and an infinitive, translated as 'it is the mark/ job / nature of x to do y'.
It is the sailor's job to navigate
which in Latin would be:
est nautae navigare.
Other examples:
est medici aegrotas sanare
est ducis urbem curare
This use of the genitive is also sometimes referred to as the genitive of characteristic.
