he judge's
duty to require; for instance, security against fraud, or for the
pursuit of a runaway slave, or (in default) for payment of his value.

2 Those are praetorian, which the praetor is bound to exact simply in
virtue of his magisterial functions; for instance, security against
apprehended damage, or for payment of legacies by an heir. Under
praetorian stipulations we must include also those directed by the
aedile, for these too are based upon jurisdiction.

3 Conventional stipulations are those which arise merely from the
agreement of the parties, apart from any direction of a judge or of the
praetor, and which one may almost say are of as many different kinds as
there are conceivable objects to a contract.

4 Common stipulations may be exemplified by that by which a guardian
gives security that his ward's property will not be squandered or
misappropriated, which he is sometimes required to enter into by the
praetor, and sometimes also by a judge when the matter cannot be managed
in any other way; or, again, we might take the stipulation by which an
agent promises that his acts shall be ratified by his principal.

TITLE XIX. OF INVALID STIPULATIONS

Anything, whether movable or immovable, which admits of private
ownership, may be made the object of a stipulation; 1 but if a man
stipulates for the delivery of a thing which either does not or cannot
exist, such as Stichus, who is dead but whom he though alive, or an
impossible creature, like a hippocentaur, the contract will be void.

2 Precisely the same principles applies where a man stipulates for the
delivery of a thing which is sacred or religious, but which he thought
was a subject of human ownership, or of a thing which is public, that is
to say, devoted in perpetuity to the use and enjoyment of the people
at large, like a forum or theatre, or of a free man whom he thought a
slave, or of a thing which he is incapable of owning, or which is
his own already. And the fact that a thing which is public may
become private property, that a free man may become a slave, that the
stipulator may become capable of owning such and such a thing, or that
such and such a thing may cease to belong to him, will not avail to
merely suspend the force of the stipulation in these cases, but it is
void from the outset. Conversely, a stipulation which originally
was perfectly good may be avoided by the thing, which is its object,
acquiring any of the characters just sp