There are two key assumptions relied on by courts to explain and justify their work in statutory interpretation. It is not that there are only these two rules.There are some other rules that are known as cannons of construction and the presumptions to the statute.However, it shall be useful to refer to to the two rules as stated above.
Plain Meaning Rule
One is the assumption that meaning inheres in legislative texts and that at least some of the time meaning is "plain" -- that is, clear and certain, not susceptible of doubt. This assumption is the necessary basis for the plain meaning rule.The plain meaning says that if the meaning of a legislative text is plain, the court may not interpret it but must simply apply it as written. The court may to resort to the rules and techniques of interpretation only if the text is ambiguous.
This rule presupposes that there is an important difference between the first-impression meaning of a text and post-interpretation meaning. First impression meaning is meaning that spontaneously comes to mind when a person reads a text relying on nothing but the text and her own linguistic competence. Post-interpretation meaning is meaning constructed by a person through interpretation, by relying on factors other than the text itself -- factors like the imagined purpose of a text, or its possible consequences, or extrinsic aids like legislative history. According to the plain meaning rule, when a person sets out to resolve a dispute about the correct interpretation of a legal text, the first thing he must do is read the text and form an impression of its meaning based on reading alone. He must then judge whether this meaning is plain. A text has a plain meaning if a competent reader would judge, on the basis of reading alone, that her first impression meaning is the only meaning the text can plausibly bear. A text is ambiguous if a competent reader could plausibly read it in more than one way.
Courts generally follow this rule for interpreting statues i.e. when the meaning is plain the only task before the court is to apply this as the legislative intent is clear and need not be derived or interpreted.
The other assumption is that legislatures have intentions when they enact legislation and these intentions are knowable by courts when called on to interpret legislation. This assumption is the necessary basis for the doctrine of fidelity to legislative intent. . The assumptions tell us that interpretation is rooted in something definite -- the text -- which has been fixed once and for all by the legislature We know that the rule of law is legitimate because law is made by the elected representatives of the people, at the end of a democratic process. We know that the rule of law is fair and efficient because law is issued in the form of texts which are fixed and stable, published in advance and applied equally to everyone under the supervision of the courts. Finally, we know that courts are well placed to supervise the application of law because of their impartiality and their interpretive expertise.