The rule ‘expressio unis est exclusion alterius’ means that, the express mention of one thing is the exclusion of other.
Where things are specifically included in list and others have been excluded it means that all others have been excluded. However, sometimes a list in a statute is illustrative, not exclusionary. This is usually indicated by a word such as “includes” or “such as”. Thus a statute granting certain rights to “police, fire, and sanitation employees” would be interpreted to exclude other public employees not enumerated from the legislation. This is based on presumed legislative intent and where for some reason this intent cannot be reasonably inferred the Court is free to draw a different conclusion.
The general meaning of “expression of one thing is the exclusion of another” is also known as the negative implication rule. This rule assumes that the legislature intentionally specified one set of criteria as opposed to the other. Therefore, if the issue to be decided addresses an item not specifically named in the statute, it must be assumed the statute does not apply.