the parties themselves. A rule which can
take effect against the judicially known will of the parties is not a
rule of construction or interpretation, but a positive rule of law.
However artificial some rules of construction may seem, this test will
always hold. In modern times the courts have avoided laying down new
rules of construction, preferring to keep a free hand and deal with each
case on its merits as a whole. It should be observed that the fulfilment
of a contract may create a relation between the parties which, once
established, is governed by fixed rules of law not variable by the
preceding agreement. Marriage is the most conspicuous example of this,
and perhaps the only complete one in our modern law.

Evidence.

There are certain rules of evidence which to some extent guide or
restrain interpretation. In particular, oral testimony is not allowed to
vary the terms of an agreement reduced to writing. This is really in aid
of the parties' deliberate intention, for the object of reducing terms
to writing is to make them certain. There are apparent exceptions to the
rule, of which the most conspicuous is the admission of evidence to show
that words were used in a special meaning current in the place or trade
in question. But they are reducible, it will be found, to applications
(perhaps over-subtle in some cases) of the still more general principles
that, before giving legal force to a document, we must know that it is
really what it purports to be, and that when we do give effect to it
according to its terms we must be sure of what its terms really say. The
rules of evidence here spoken of are modern, and have nothing to do with
the archaic rule already mentioned as to the effect of a deed.

Performance.

Every contracting party is bound to perform his promise according to its
terms, and in case of any doubt in the sense in which the other party
would reasonably understand the promise. Where the performance on one or
both sides extends over an appreciable time, continuously or by
instalments, questions may arise as to the right of either party to
refuse or suspend further performance on the ground of some default on
the other side. Attempts to lay down hard and fast rules on such
questions are now discouraged, the aim of the courts being to give
effect to the true substance and intent of the contract in every case.
Nor will the court hold one part of the terms deliberately agreed to
more or less ma