405. The voluntary agent after a passive verb is expressed by the ablative with ā or ab.
Laudātur ab hīs, culpātur ab illīs (Hor. S. 1.2.11)
He is praised by these, blamed by those.
Ab animō tuō quidquid agitur id agitur ā tē. (Tusc. 1.52)
Whatever is done by your soul is done by yourself.
Ā fīliīs in iūdicium vocātus est. (Cat. M. 22)
He was brought to trial by his sons.
cum ā cūnctō cōnsessū plausus esset multiplex datus (id. 64)
when great applause had been given by the whole audience
nē virtūs ab audāciā vincerētur (Sest. 92)
that valor might not be overborne by audacity
(Audācia is, in a manner, personified here.)
Note 1— This construction is developed from the Ablative of Source. The agent is conceived as the source or author of the action.
Note 2— The Ablative of the Agent (which requires ā or ab) must be carefully distinguished from the Ablative of Instrument, which has no preposition (§ 409).
slain by a sword
occīsus ab hoste
slain by an enemy
Note 3— The Ablative of the Agent is commonest with nouns denoting persons, but it occurs also with names of things or qualities when these are conceived as performing an action and so are partly or wholly personified, as in the last example under the rule.
a. The Ablative of the Agent with ab is sometimes used after intransitive verbs that have a passive sense.
perīre ab hoste
to be slain by an enemy
b. The personal agent, when considered as instrument or means, is often expressed by per with the accusative, or by operā with a genitive or possessive.
Ab explōrātōribus certior factus est. (B. G. 1.21)
He was informed by scouts (in person).
Per explōrātōrēs Caesar certior factus est (id. 1.12)
Cæsar was informed by (means of) scouts.
ēlautae operā Neptūnī (Plaut. Rud. 699)
washed clean by the services of Neptune
Nōn meā operā ēvēnit. (Ter. Hec. 228)
It hasn't happened through me (by my exertions).
cf. êius operā (B. G. 5.27)
Note 1— The Ablative of Means or Instrument is often used instead of the Ablative of Agent, especially in military phrases.
Haec excubitōribus tenēbantur. (B. G. 7.69)
These (redoubts) were held by means of sentinels.
Note 2— An animal is sometimes regarded as the means or instrument, sometimes as the agent. Hence both the simple ablative and the ablative with ab occur.
equō vehī [Notab equō]
to ride on horseback
(be conveyed by means of a horse)
clipeōs ā mūribus esse dērōsōs (Div. 1.99)
that the shields were gnawed by mice
For the Dative of the Agent with the gerundive, see § 374.