As Se-Shauna Wheatle acknowledges throughout Principled Reasoning in Human Rights Adjudication, there is significant debate concerning all aspects of the rule of law and the separation of powers, including what role, if any, they should have in judicial reasoning. One only needs to look at the ongoing debate as to the role and powers of the courts in the UK following the mooted repeal of the Human Rights Act 1998 (HRA) to see the everyday relevance of this discussion. Rather than worsen that uncertainty, Principled Reasoning aims to be a companion to the broader normative debate. Wheatle pursues this objective in two ways: first, by pragmatic analysis of the rule of law and the separation of powers in fundamental human rights adjudication in four jurisdictions which share common law and colonial histories; secondly, by thematic analysis of such principles in judicial decision-making. Wheatle's approach provides a timely insight into “the ways in which the rule of law and the separation of powers are used as means of legitimating judicial decisions in rights cases, and the conceptual and constitutional difficulties that arise from this practice”, demonstrating that the debate surrounding the role and scope of implied constitutional principles is far more nuanced than certain commentary suggests.

Principled Reasoning has two parts. Part I examines how courts in the UK, Canada, Australia and the Commonwealth Caribbean have conceptualised the rule of law and separation of powers. After clarifying the terms of discussion and establishing that the rule of law and separation of powers are central to common law constitutionalism (ch. 2), Wheatle examines the case law of the chosen jurisdictions in earnest in chs. 3 and 4. Wheatle canvasses the various judicial conceptions of the rule of law and applications of the separation of powers, suggesting that applications of the rule of law can be divided into two categories: formal conceptions (i.e. rule by law) and substantive conceptions which speak to the content of the law. Substantive conceptions can be subcategorised as either legalistic (those mainly concerned with the content of the law) or “more deeply valorised” (those encompassing broader requirements of law's content). The rest of ch. 3 considers the formal, legalistic and “valorised” conceptions of the rule of law in turn, and concludes with the observation that “the rule of law ideal implicates the substance of the law through either a legalistic or valorised model”. This is at odds with the dominant discourse amongst legal theorists, which emphasises formalistic conceptions of the rule of law. Despite its focus on the four jurisdictions covered, Wheatle's analysis will be helpful to a wider audience, especially in territories that have a common law heritage.

The existence and importance of the separation of powers have been disputed for years. However, Wheatle shows that the principle is ardently accepted by courts in Westminster-style systems, which is a helpful grounding for her comparative discussion. Wheatle shows that courts across the selected jurisdictions frequently draw on the principle to reinforce judicial independence and legitimacy, including in jurisdictions in which parliamentary supremacy has given way to constitutional supremacy (as in the Commonwealth Caribbean). Rejecting the contention that courts adopt a purely theoretical concept of the separation of powers, Wheatle concludes ch. 4 showing how the principle is used in human rights cases. She demonstrates that the separation of powers principle is used by courts to reinforce judicial independence (and restraint) which, in turn, gives judicial decisions on governmental action coercive power.

Part II of the book explores the functions of the rule of law and the separation of powers in the jurisdictions surveyed. Both principles have been used as interpretative aids (ch. 5). Wheatle argues that courts do so in three ways: as a “distancing device”, as common ground in normative disputes and to secure institutional respect. According to Wheatle, each of these uses is motivated by a desire to bolster the courts’ legitimacy, by buffering their decisions from claims that they were influenced by personal values or opinions. Wheatle's citation of R. (on the Application of Nicklinson) v Ministry of Justice [2014] UKSC 38, [2014] H.R.L.R. 17 in this connection may undermine her findings. Arguably, the failure of the majority Supreme Court in that case to exercise its supervisory jurisdiction as conferred by Parliament via the HRA is evidence that implied principles are ineffective, particularly in “hard” human rights cases. The deference shown by the majority arguably demonstrates an abdication of Wheatle's claimed “duty” of the courts to decide “hard” cases. But implied principles can secure institutional respect, as Wheatle shows by reference to cases on ouster clauses. The effective use of the rule of law and separation of powers as interpretive aids through which courts’ supervisory powers have been secured while reinforcing the supremacy of Parliament is well shown by A. v HM Treasury [2010] UKSC 2, [2010] H.R.L.R. 15. Although not discussed by Wheatle, aspects of the reasoning in that case aptly illustrate the Supreme Court's use of implied principles to secure legitimacy and respect (see Lord Phillips's observation that in invalidating the executive's orders, the court was not displacing the will of Parliament but instead vindicating the primacy of Parliament in determining in what circumstances fundamental rights may be restricted).

Wheatle continues Part II's examination of the functions of implied principles by exploring the ways in which they can also serve as independent grounds of invalidation – perhaps the most contentious of the potential functions of implied constitutional principles (ch. 6) – and their role in justifying reliance on foreign jurisprudence (ch. 7). The analysis undertaken in ch. 6 helpfully demonstrates that debate surrounding the utilisation of implied constitutional principles in the invalidation of legislation is more nuanced than existing academic debate suggests, and highlights the superficiality of claims that Lord Steyn's reasoning in R. (on the application of Jackson) v Attorney General [2005] 1 A.C. 262, [2005], to the effect that the power of a UK court to invalidate legislation exists in “exceptional circumstances”, should simply be rejected. In ch. 7, Wheatle contends that, in post-colonial settings, the citing of foreign jurisprudence (particularly that of the former colonial power) reflects a connection between transition and continuity which adds legitimacy to the institutions of newly independent states. References to foreign jurisprudence are also a means of distilling a common solution to a common problem, a tactic which could potentially be used beyond the territories which Wheatle analyses.

The final substantive chapter of Principled Reasoning (ch. 8) draws together both parts to demonstrate how the book can be used in future normative debate concerning the legitimacy of reliance on implied constitutional principles in rights adjudication. More specifically, Wheatle argues that the utilisation of implied constitutional principles is not monolithic but, rather, “manifests in a variety of ways” along a “spectrum of functions” and, “suggests a path forward for systematic normative analysis of implied principle techniques” including as a means of defeating “unconstitutional constitutional amendments”. While other uses of implied constitutional principles are also contentious, the role of implied constitutional principles faces the greatest opposition where the repeal of legislation is proposed, particularly in Westminster systems dominated by the concept of parliamentary supremacy. Wheatle provides us with a basis to argue that implied constitutional principles may justify the repeal of legislation. Although Wheatle's review acknowledges that the case law from the surveyed jurisdictions “does not lead inexorably to judicial support for doctrines that would permit the judicial defeat of constitutional amendments”, her analysis shows a depth to the issue which much existing academic commentary does not show.

Principled Reasoning offers insights that are timely given that the repeal of the HRA is being considered. Wheatle examines how the House of Lords and the UK Supreme Court have employed implied constitutional principles in deciding human rights cases. She clarifies how courts could engage in judicial review of legislation (a power which so far deriving exclusively from the HRA in England and Wales) and protect individual rights through implied constitutional principles. It is difficult to overstate the importance of that exercise, given the history of courts’ relying on implied constitutional principles – particularly the rule of law – to examine the validity of legislation in the context of rights protection. Wheatle provides a vital context to the broader normative debate and hope to those who fear the unravelling of the protections offered by the HRA.

Since the rule of law and separation of powers are contested, it is inevitable that some readers will disagree with Wheatle's analysis and conclusions. For instance, while Wheatle defines “fundamental human rights” cases as those “in which it is alleged that the act or omission of a government organ violates an individual's liberty”, she does not define “liberty”. Public lawyers will appreciate the difficulty of defining “liberty”. Wheatle's focus on liberty cases in Part I causes one to ask why she does not define the term. More curiously, despite Wheatle's focus on the distinction between “fundamental human rights” and human rights generally, the case analysis in Principled Reasoning is not confined to “fundamental human rights” cases: it includes cases which concern human rights more generally (e.g. p. 112). Other aspects of Wheatle's methods or the parameters of her analysis may also trouble readers. However, fixating on Wheatle's methods would risk obscuring the utility of Principled Reasoning. For example, the references to general human rights do not undermine Wheatle's analysis. They reinforce the role of implied constitutional principles in human rights adjudication.

If readers take nothing else from Principled Reasoning, they will be left in no doubt that concentrating on the normative debate runs the risk of obscuring key variations in the “sources, methods of implication, and functions of implied constitutional principles across a range of textual and institutional backgrounds”. The question, as Principled Reasoning illustrates, is which if any of the range of potential uses of implied constitutional principles is legitimate. Readers will not find an answer to that query in Principled Reasoning. They will nevertheless be better placed to critique the practice of utilising implied constitutional principles in light of Wheatle's empirical and comparative analysis. Her book is a welcome addition to the debate, and a key new release which complements the other titles in the Hart series.