Like a signal No 8 typhoon, the national security law directly hit Hong Kong just before midnight on July 1, leaving us to pick up the pieces. One of those pieces is its interpretation. Some have asked why bother as it is like other Chinese laws – vague and open to manipulation through interpretation by the authorities. Only the National People’s Congress Standing Committee appears to have the power to interpret the law . Let the political struggle continue, they say. As a law professor and practitioner, I find such a defeatist attitude unhelpful. Cases under the new law have commenced . Lawyers need to advise on it and courts must apply it in adjudicating cases. The law is upon us and we cannot sit idle in fear, waiting for some authority to tell us what it means. In affirming our autonomy, questions of interpretation should be carefully considered on our own in accordance with existing legal practices and principles. The national security law has been added to Annex III of the Basic Law by the NPC Standing Committee. Annex III national laws are to be “applied locally” – that is, by reference to local circumstances and standards. Hong Kong judges and practitioners work in a common law legal system, having been educated and trained in the common law tradition. Few will have obtained mainland legal qualifications or studied more than an introductory course on mainland law. Few Hong Kong lawyers have experience practising law on the mainland as admission and practice is strictly regulated. When questions of mainland law arise in Hong Kong litigation, like questions of foreign law, expert evidence of mainland law is required. This bifurcation of legal systems is a direct and intended consequence of the Basic Law, which directs that the “ socialist system and policies shall not be practised ” in Hong Kong. As recognised by our Court of Final Appeal, many Basic Law articles are devoted to establishing Hong Kong’s separate system , of which guarantees of rights and freedoms lie at its heart. Is Chinese legal expertise needed because the law migrated from existing Chinese laws? While some provisions were obviously taken from Chinese criminal law, many were specially drafted for the Hong Kong context. Take for example the crimes of secession and subversion . The Criminal Law of the People’s Republic of China provides for these crimes simply as “splitting the state or undermining unity of the country” and “subverting the state power or overthrowing the socialist system”. By contrast, Articles 20 and 22 of our new law provide for these offences in greater detail, with reference to matters relevant to Hong Kong. Not being a cut and paste of mainland law, it cannot be assumed mainland officials or scholars will necessarily have answers to interpretative questions. How Beijing’s national security legislation endangers Hong Kong rule of law Perhaps the best guidance to interpreting the new law is our courts’ approach to interpreting the Basic Law , another national law specially designed for Hong Kong. The Court of Final Appeal’s common law approach is an objective exercise aimed at identifying “the meaning borne by the language when considered in the light of its context and purpose”. Provisions are not to be given “a meaning which the language cannot bear”. The common law approach gives less weight, if any, to external considerations such as post-enactment statements by officials. Fundamental to our approach is the principle of legal certainty, which has two aspects. All law must be adequately accessible and formulated with sufficient precision to enable a person to regulate their conduct. Our courts have shown an indomitable spirit in clarifying seemingly vague laws with the help of purpose and context. As for accessibility, the rush to enforce this new law in the absence of an official English text approved by the NPC Standing Committee raises real questions of whether the law is adequately accessible. While the Basic Law and national security law have a hierarchical relationship to ordinary laws, the latter was intended to be embedded in Hong Kong’s existing criminal laws as an integrated legal regime and not a self-enclosed code. This is another reason to interpret terms and concepts in the new law consistently with those used in existing Hong Kong laws. For example, it would be odd if “tried on indictment” in Article 41 of the new law had a different meaning from the one in the Criminal Procedure Ordinance. Well-established common law principles applicable to criminal laws should also be applied. The presumption of mens rea holds that if legislation is silent or ambiguous on whether a mental element of intention or knowledge need be proven, it must be presumed such element is essential unless there are good reasons to displace the presumption. This principle will be critical to understanding the terrorism and collusion offences for which the mens rea elements are unclear. The presumption is related to two other important principles recognised by the Court of Final Appeal. It is a principle of statutory interpretation that ambiguity in a penal statute is resolved in favour of the accused. There is also a principle of legality , that “human rights and fundamental principles of law, even where derogable, cannot be overridden except by express words or necessary implication”. If the human rights clause in Article 4 of the new law is to be more than window-dressing, the principle of legality must be allowed to do its work. It is neither practical nor necessary to obtain expert evidence or a NPC Standing Committee interpretation each time an ambiguity arises in a case. For courts to adjudicate cases, their authority to interpret the applicable law must be implied. Respect for autonomy and Hong Kong’s authority to discharge its duties under the new law requires the NPC Standing Committee exercise restraint in its power of interpretation. Excessive intervention will destroy the separate systems model and cause great legal instability. Simon Young Ngai-man is professor and an associate dean of the Faculty of Law at the University of Hong Kong and a practising barrister