constitution. Although Article 4 of the Singapore Constitution expressly declares that it is the supreme law of the land and the Constitution appears to satisfy Dicey's criteria, the view has been taken that it may not be supreme in practice and that Singapore's legal system is de facto characterized by parliamentary sovereignty. Codification Dicey's first legal criterion for a constitution to be regarded as supreme is that it must be written. This requirement is necessary for the precise identification of constitutional provisions, which makes it more convenient for Parliament to make constitutional amendments, and provides the judiciary with a basic text against which to determine the constitutionality of any ordinary legislation. Without a written constitution, judicial review would almost be counter to the doctrine of separation of powers as judges would get to decide the contents and wording of the Constitution. In Marbury v. Madison, the US Supreme Court held that "the powers of the legislature are defined and limited; and that those limits may not be mistaken, or forgotten, the Constitution is written". However, in Singapore not all legal rules having constitutional effect appear to be part of the Constitution. For example, white papers that contain quasi-constitutional principles would be extra-constitutional documents. By issuing such white papers, the Government may also trying to set guidelines on how the Constitution should be interpreted. Jaclyn Neo and Yvonne Lee view such documents as diluting the Constitution and blurring the line between constitutional law and ordinary legislation. Rigidity The second legal criterion is that the constitution must be rigid. This is important to ensure that constitutional provisions can only be changed by an authority that is higher in status than the ordinary legislative body existing under the Constitution. However, rigidity does not mean that the Constitution is completely immutable. If the Constitution is static, the nation's political development may be stunted. Instead, rigidity of the Constitution merely contemplates that compared to ordinary legislation, the Constitution should be more difficult to amend. Different amendment procedures apply to different parts of the Constitution. This is discussed in detail below. Most of the Articles of the Constitution may be amended by a bill enacted by Parliament if there is at least a supermajority of two-thirds of all elected MPs voting in favour of the bill during its Second and Third Readings in Parliament. Since ordinary bills only need to be approved by at least a