Sometimes referred to in short form as expressio unius.
In Rodaro, Justice, at ¶856, defined the Latin maxim expressio unius est exclusio alterius as follows:
"... a maxim of interpretation meaning that the expression of one thing is the exclusion of the other. When certain persons or things are specified in a law, contract or will, an intention to exclude all others from its operation may be inferred. In this case, the reference to the assignment to a financial institution excludes assignment to any other entity."
In Dorval, Justice Cameron expressed it as:
"... to express one thing is to exclude another."
In Transpacific, Justice Lysyk described it as follows:
"The principle ... expressio unius est exclusio alterius: the express mention of one or more things of a particular class may be regarded as impliedly excluding others."
Although the doctrine is useful in determining the extents of contracts, it is also an important principle in the construction of statutes. In her book on the topic, jurist Ruth Sullivan wrote:
"One of the so-called maxims of statutory interpretation is expressio unius est exclusio alterius: to express one thing is to exclude another.
"The maxim reflects a form of reasoning that is widespread and important in interpretation .... the a contrario argument ... negative implication ..implied exclusion ...
"An implied exclusion argument lies whenever there is reason to believe that if the legislature had meant to include a particular thing within the ambit of its legislation, it would have referred to that thing expressly. Because of this expectation, the legislature’s failure to mention the thing becomes grounds for inferring that it was deliberately excluded. Although there is no express exclusion, exclusion is implied."
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