adfui scribendo, I was present at the writing.

3.  Accusative.  The Accusative of the Gerund is used only with Prepositions, chiefly ad and in to denote purpose; as,—­

    homo ad agendum natus est, man is born for action.

4.  Ablative.  The Ablative of the Gerund is used—­

  a) Without a Preposition, as an Ablative of Means, Cause, etc. (see Sec.
  218, 219); as,—­

    mens discendo alitur et cogitando, the mind is nourished by learning
    and reflection.

    Themistocles maritimos praedones consectando mare tutum reddidit,
    Themistocles made the sea safe by following up the pirates.

  b) After the prepositions a, de, ex, in; as,—­

    summa voluptas ex discendo capitur, the keenest pleasure is derived
    from learning;

    multa de bene beateque vivendo a Platone disputata sunt, there was
    much discussion by Plato on the subject of living well and happily.

5.  As a rule, only the Genitive of the Gerund and the Ablative (without a preposition) admit a Direct Object.

Gerundive Construction instead of the Gerund.

339. 1.  Instead of the Genitive or Ablative of the Gerund with a Direct Object, another construction may be, and very often is, used.  This consists in putting the Direct Object in the case of the Gerund (Gen. or Abl.) and using the Gerundive in agreement with it.  This is called the Gerundive Construction.  Thus:—­

         GERUND CONSTRUCTION.  GERUNDIVE CONSTRUCTION.
  cupidus urbem videndi, desirous of cupidus urbis videndae;
  seeing the city.
  delector oratores legendo, I am delector oratoribus legendis
  charmed with reading the orators.

2.  The Gerundive Construction must be used to avoid a Direct Object with the Dative of the Gerund, or with a case dependent upon a Preposition; as,—­

locus castris muniendis aptus, a place adapted to fortifying a camp;

    ad pacem petendam venerunt, they came to ask peace;

    multum temporis consumo in legendis poetis, I spend much time in
    reading the poets.

3.  In order to avoid ambiguity (see Sec. 236, 2), the Gerundive Construction must not be employed in case of Neuter Adjectives used substantively.  Thus regularly—­

    philosophi cupidi sunt verum investigandi, philosophers are eager for
    discovering truth (rarely veri investigandi);

    studium plura cognoscendi, a desire of knowing more (not plurium
    cognoscendorum).

4.  From the nature of the case only Transitive Verbs can be used in the Gerundive construction; but utor, fruor, fungor, potior (originally transitive) regularly admit it; as,—­

    hostes in spem potiundorum castrorum venerant, the enemy had conceived
    the hope of gaining possession of the camp.