The principle here is that when particular things are mentioned or listed in the provisions of an Act, other specific things not mentioned are excluded.

For example, if a section specifically refers to 'cats and dogs' it does not apply to other animals such as rabbits and parrots.

Of course it can always be argued that a legislature's failure to mention further specific things was due to error or inadvertence rather than deliberate policy. Thus 'expressio unius' is best applied when the circumstances indicate the legislature intended the provision to apply only to the things mentioned.

The principle of 'expressio unius' is generally used to support interpretations formed in other ways, rather than to produce a result on its own.