The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Ablative of Agent.
405. The Voluntary Agent after a passive verb is
expressed by the Ablative with á or ab: -
- laudátur ab hís,
culpátur ab illís (Hor. S. i. 2. 11),
he is praised by these, blamed by those.
- ab animó tuó quidquid
agitur id agitur á té (Tusc. i. 52),
whatever is done by your soul is done by yourself.
- á filiís in
iúdicium vocátus est (Cat. M. 22), he was
brought to trial by his sons.
- cum á cúnctó
cónsessú plausus esset multiplex datus (id. 64), when great applause had been given by the whole audience.
- né virtús ab
audáciá vincerétur (Sest. 92), that
valor might not be overborne by audacity. [AudáActa is in a manner personilled.]
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 á or ab) must be carefully distinguished from the
ablative of instrument, which has no preposition (§400). Thus -
occísus gladió, slain by a sword; but,
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 persomfied, 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: -
- períre 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
operá with a genitive or
explórátóribus certior factus est (B. G. i. 21), he was informed by scouts (in person). But,
explórátórés Caesar certior factus
est (id. i. 12), Caesar was is formed by (means of)
- élautae operá
Neptúní (Plaut. Rud. 699), washed clean by
the services of Neptune.
- nón meá operá
événit (Ter. Hec. 228), it hasn't happened
through me (by my exertions). [Cf. êius operá, B. G. v. 27.]
NOTE 1: The ablative of means or instrument is often used
instead of the ablative of agent, especially in military phrases: as, -
tenébantur (B. G. vii. 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: -
- equó vehí, to ride on
horseback (be conveyed by means of a horse). [Not ab equó.]
- clipeós á mmúribus
esse dérósós (Div. i. 99), that the
shields were gnawed by mice.
For the Dative of the Agent with the Gerundive, see §374.