On the Interior Senses
Latin Grammar: Neuter Gerundives
Deinde est vis aestimationis, quae est vis ordinata in summo mediae concavitatis cerebri, apprehendens intentiones non sensatas quae sunt in singulis sensibilibus, sicut vis quae est in ove diiudicans quod ab hoc lupo est fugiendum, et quod huius agni est miserendum; videtur etiam haec vis operari in imaginatis compostionem et divisionem.
When the gerundive is used with a form of sum, the resulting clause expresses necessity. So above we have ab hoc lupo est fugiendum, "this wolf must be fled (from)." The neuter form used above makes the construction impersonal, but an agent in the dative can be added as well: ab hoc lupo mihi est fugiendum, "I must flee this wolf." Both transitive and intransitive verbs can use the construction.