In summary:
(1) the primary duty of the court in construing an instrument is to endeavour to discover the intention of the parties as embodied in the words they have used in the instrument;
(2) it is the objectively ascertained intention of the parties, as it is expressed in the instrument, that matters, not the parties' subjective intentions. The meaning of the terms of a contractual document is to be determined by what a reasonable person would have understood the terms to mean;
(3) the objectively ascertained purpose and objective of the transaction may be taken into account in construing the instrument. That may invite attention to the genesis of the transaction, its background and context;
(4) the apparent purpose or object can be inferred from the express and implied terms of the contract, and from any admissible evidence of surrounding circumstances; and
(5) an instrument can be construed so as to avoid it making commercial nonsense or giving rise to commercial inconvenience. However it must be borne in mind that business common sense may be a topic on which minds may differ.
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A contract, like a statute, must be constructed as a whole. A construction that makes the various parts of an instrument harmonious is preferable. If possible, each part of an instrument should be construed so as to have some operation.
On the proper construction of a contract, evidence of surrounding circumstances is not admissible unless the court determines that the contract is ambiguous or susceptible of more than one meaning.
Ambiguity is not confined to lexical, grammatical or syntactical ambiguity. It is enough if the instrument is susceptible of more than one meaning or if the scope or applicability of the contract is doubtful.
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There are limits on the extent to which surrounding circumstances can influence the proper construction of an instrument. Reliance on the surrounding circumstances must be tempered by loyalty to the text of the instrument.
There are also limits on the kind of evidence which is admissible as background to construction of a contract, and the purposes for which it is admissible. Evidence of prior negotiations is admissible for some purposes but not for others. Insofar as such evidence establishes objective background facts known to the parties or the genesis, purpose or object of the transaction it is admissible. Insofar as it consists of statements and actions of the parties reflective of their actual intentions and expectations it is inadmissible. Such statements reveal the terms of the contracts which the party intended or hoped to make. They are superseded by, or merged into, the contract.