When determining the meaning of particular words the courts will make certain presumptions about the law. If the statute clearly states the opposite, then a presumption will not apply and it is said that the presumption is rebutted. The main presumptions are:
A presumption against change in the common law.
It is assumed that the common law will apply unless Parliament has made it plain in the Act that the common law has been altered.
A presumption that mens rea (‘guilty mind’) is required in criminal cases.
Mens rea is one of the elements that has to be proved for a successful criminal prosecution. There is a common law rule that no one can be convicted of a crime unless it is shown they had the required intention to commit it.
A presumption that the Crown is not bound by any statute unless the statute expressly says so.
A presumption that a statute does not apply retrospectively. No statute will apply to past happenings. Each statute will normally only apply from the date it comes into effect. This is, however, only a presumption and Parliament can choose to pass a statute with retrospective effect. This must, however, be expressly stated in the statutes, for example, the War Damage Act 1965, the War Crimes Act 1991 and the Adoption Act 1976.
The secondary aids are rules of language, intrinsic and extrinsic aids.