The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Dative of the Agent.
374. The Dative of the Agent is used with the
Gerundive to denote the person on whom the necessity rests: -
- haec vóbís
próvincia est défendenda (Manil. 14), this
province is for you to defend (to be defended by you).
- mihi est púgnandum, I have to fight
(i.e. the need of fighting is to me: cf. mihi est liber, I have a book, § 373. N.).
a. This is the regular way of expressing
the agent with the Second or Passive Periphrastic Conjugation
NOTE 1: The Ablative of the Agent with ab (§ 405)
is sometimes used with the Second Periphrastic Cobjugation when the Dative
would be ambiguous or when a stronger expression is desired: -
- quibus est á
vóbís cónsulendam (Manil. 6), for whom
you must consult. [Here two datives, quibus and vóbís, would have been
- rem ab omnibus vóbís
próvidendam (Rabir. 4), that the matter must be attended
to by all of you. [The dative might mean for all of you.]
NOTE 2: The Dative of the Agent is either a special case of
the Dative of Possession or a development of the Dative of Reference