In Singapore, the rule of law doctrine has been the topic of considerable disagreement and debate, largely through differing conceptions of the doctrine. These conceptions can generally be divided into two categories developed by legal academics, the "thin", or formal, conception and the "thick", or substantive, conception of the rule of law. The thin conception, often associated with the legal scholars Albert Venn Dicey and Joseph Raz, advocates the view that the rule of law is fulfilled by adhering to formal procedures and requirements, such as the stipulations that all laws be prospective, clear, stable, constitutionally enacted, and that the parties to legal disputes are treated equally and without bias on the part of judges. While people subscribing to the thin conception do not dismiss the importance of the content of the law, they take the view that this is a matter of substantive justice and should not be regarded as part of the concept of the rule of law. On the other hand, the thick conception of the rule of law entails the notion that in addition to the requirements of the thin rule, it is necessary for the law to conform to certain substantive standards of justice and human rights.
A thin conception of the rule of law has generally been adopted by the Singapore Government and Members of Parliament from the ruling People's Action Party, as evidenced from the views expressed during a 1999 parliamentary debate on the rule of law. However, a thicker conception was evinced by the Minister for Law in a speech made during the 2009 seasonal meeting of the New York State Bar Association's International Section in Singapore. On the other hand, a thick conception of the rule of law that encompasses human rights has been adopted by a number of Government critics, including opposition politicians, and foreign and international organizations such as Human Rights Watch, Lawyers' Rights Watch Canada, and the World Justice Project.
Some of the key principles associated with the thin conception of the rule of law include judicial independence, natural justice, the availability of judicial review, and the accessibility of justice. Judicial independence in Singapore is safeguarded by various constitutional provisions and legal rules, though some commentators have highlighted certain events as suggesting a lack of judicial independence. One incident in the 1980s involving the transfer of the Senior District Judge to the Attorney-General's Chambers following a decision he made which was favourable to an opposition politician was investigated by a commission of inquiry and found not to be due to executive interference. In Singapore, natural justice is generally regarded as a procedural rather than a substantive concept. The process of judicial review involves the review of executive actions for compliance with administrative law rules, and of executive and legislative acts for unconstitutionality in light of the doctrine of constitutional supremacy. The state of the courts and legal processes in Singapore are largely seen as making justice accessible for the citizenry.
For those taking a thick conception of the rule of law, the existence of the Internal Security Act (Cap. 143, 1985 Rev. Ed.) ("ISA") is widely regarded as a breach of the doctrine. The Act, which provides for detention without trial for people regarded by the executive as a risk to national security, is shielded from unconstitutionality by Article 149 of the Constitution. Although the Court of Appeal held in the 1988 case Chng Suan Tze v. Minister for Home Affairs that the courts could review the legality of detentions under the Act, the effect of the case was reversed through amendments to the Constitution and the ISA in 1989. The ISA amendments were determined to be effective by the High Court and Court of Appeal in Teo Soh Lung v. Minister for Home Affairs in 1989 and 1990 respectively. While the executive's largely unconstrained discretion to detain under the ISA has been criticized as contrary to the rule of law, the Government has justified the statute as a crucial measure of last resort for preserving security.