Not long ago, it seemed that there was little to say about the human right to freedom of thought because it was generally understood that mindreading was technically impossible. The right was regarded as largely theoretical with little real-life relevance. With recent developments in neurotechnologies, this is no longer so certain. In just a few years, there has been a surge in commercial products claiming to read and influence thoughts using electroencephalography (EEG), functional magnetic resonance imaging (fMRI), brain-machine inferences (BMI) or other neurotechnologies. Technological advancements claiming to read minds and enhance cognition have created a surge in interest in the right to freedom of thought. However, the academic literature on this human right is still slim, which makes this book highly relevant as one of the first modern authoritative texts on the topic. The Cambridge Handbook of The Right to Freedom of Thought, sympathetically edited by Patrick O’Callaghan and Bethany Shiner, is a timely guide in an emerging technological landscape.

The right to freedom of thought is set out in Article 18 of the Universal Declaration of Human Rights (UDHR). It is also in numerous other instruments including Article 18 of the International Covenant on Civil and Political Rights (ICCPR), Article 9 of the European Convention on Human Rights (ECHR), Article 10 of the Charter of Fundamental Rights of the EU (CFR) and Article 13 of the American Convention on Human Rights (ACHR). As chapters on different countries in this volume explain, the right to freedom of thought may also appear in national constitutions, ordinary legislation or case law. For example, in his chapter on freedom of thought in the US, Marc Jonathan Blitz notes that while not explicitly mentioned in the US Constitution, the US Supreme Court has found the right implicitly in several cases. In one case, the Court held that “[l]iberty presumes an autonomy of self that includes freedom of thought, belief, expression, and certain intimate conduct” where individual states “should not be a dominant presence” (Lawrence v Texas, 539 U.S. 558, 562 (2003)) (p. 303).

The Human Rights Committee in its General Comment 22 (1993) stated that Article 18 UDHR is “far-reaching and profound” and “encompasses freedom of thought on all matters”. Given its importance, it is therefore perhaps surprising that the editors of this Handbook explain in their introductory chapter, that freedom of thought “remains one of the least understood and explored rights in legal practice and scholarship” (p. 3). They set out to remedy these epistemologically lacunae in the 28 chapters that follow.

As a compendium, the Handbook examines the right to freedom of thought from two sides. The first is theoretical in nature, exploring the right’s origin, historical evolution, thorny issues of scope and interpretation, and its relevance in specific contexts. From an academic viewpoint, these chapters offer the volume’s most significant contribution to legal scholarship as they provide comprehensive and detailed analyses of definitions, applications and relation to other rights. As expected, several of the volume’s authors relate their doctrinal examinations to UN Special Rapporteur on Freedom of Religion or Belief, Ahmed Shasheed’s definition of the right: (1) freedom not to disclose one’s thoughts, (2) freedom from punishment for one’s thoughts, (3) freedom from impermissible alteration of one’s thoughts and (4) states’ obligation to provide an enabling environment for freedom of thought.

Where the arguments become most interesting in this book, however, is in the more nuanced theoretical explorations, for example on how to delineate the scope of protection between the forum internum, broadly understood to relate to the thoughts in one’s head, and the forum externum, which concerns the manifestation of thought. The distinction matters because thoughts which fall under the first forum is afforded absolute protection, while protection under the latter is qualified. To grasp the implications of this distinction in relation to the ECHR, Shiner draws on European Court of Human Rights jurisprudence. Shiner goes on to write that “[w]hen it comes to the manifestation of beliefs, it is not enough for the action or conduct to be motivated by the belief; the action or conduct must ‘actually’ express the belief” (p. 149). This leads to questions regarding the line between the qualified protection of the right to freedom of thought as manifestation and the right to freedom of expression (art. 19 UDHR). Christoph Bublitz, in his historical chapter, argues that Articles 18 (freedom of thought) and 19 (freedom of expression) UDHR “should not be understood in isolation, protected side by side, but as interwoven continuations, sharing freedom of thought as their common foundation” (p. 26). This interpretation is also in some instances reflected in law, which arguably undermines the right to freedom of thought as a free-standing right. For example, in the chapter on the ACHR, Cláudio de Oliveira Santos Colnago and Shiner explain how the freedom of thought and the freedom of expression are both protected in Article 13 ACHR, leading the courts to interpreting these as interdependent. The right to freedom of thought is also frequently tangled with the right to privacy. In her chapter on the ECHR, Felicitas Benzinger notes that the absolute right to freedom of thought in Article 9(1) ECHR can bolster the qualified right to respect for private life in Article 8 ECHR, in cases concerning the right to mental privacy and mental integrity.

The issue of mental privacy and mental integrity leads to another theme which runs through the book, questioning whether the use of manipulation could interfere with the freedom of thought. In their chapter on online manipulation, Nina Keese and Mark R. Leisner argue that use of deceptive design techniques such as concealment or obfuscation may count as interference because they are used with the intention of making someone change their mind (p. 366). To Keese and Leisner, the use of these techniques constitutes an infringement of the freedom of thought because they violate the fundamental values of autonomy and privacy.

In contrast, other authors take a different view, distinguishing commercial and political manipulation or manipulation which can be seen to interfere with one’s identity. In her chapter on Germany, Nora Hertz writes that “[o]nly extreme forms of manipulation, such as indoctrination and ‘brainwashing’, or compulsion to reveal one’s beliefs, conscience and (non-) religious conviction would constitute an interference” (p. 115). Thus, “[m]ore subtle ways of manipulation, such as nudging, might not constitute an interference, nor does the disclosure (‘reading’) of unconscious mental processes or emotions” (p. 115). Nevertheless, Hertz is open to suggestions that microtargeting, dark patterns and emotional recognition systems could interfere with the freedom of thought because they may infringe the right to personality under German Basic Law. It is not quite clear how she, or similarly Keese and Leisner, arrive at this point by referencing EU laws such as the EU AI Act or the Digital Services Act. Rather than treat the freedom of thought as a free-standing right against manipulation, it seems that these authors are using it either to prop up other rights outside the international human rights doctrine or using the other rights as justification for their interpretation. The result is perhaps less clarity as to the scope of the right of freedom of thought than what the authors may have intended.

Shiner is also willing to push the boundaries of the analysis in a different direction. Drawing on John Stuart Mill calling for liberty of thought and feeling in On Liberty (1859), she argues that the capacity to think is integral to the right to freedom of thought, which leads her to the logical conclusion that there is also a right to access information, which may extend to the right to education. Her argument is not plucked out of thin air but builds on multiple instances where similar cases have been made. For instance, as Blitz demonstrates in the chapter on the US, Justice Douglas in Griswold v Connecticut (381 U.S. 479, 482 (1965)) found an implied right to freedom of thought in the First Amendment in the US Constitution. According to Douglas, the First Amendment on free speech included “the right to distribute, the right to receive, the right to read and freedom of inquiry” and “freedom of thought” (p. 299). What of course is notable with this line of argument is that while the theory may be there, until recently, there was very little cause for the courts to probe the extent of the right because, as demonstrated, so much of the case law primarily concerns different human rights. In many cases, the right to freedom of thought was relegated to a supporting role. The tacit question the theoretical part of the Handbook poses to the reader may therefore be – notwithstanding the advent of neurotechnologies – if there are sufficient grounds to treat the right to freedom of thought as free-standing. Given the many illustrations of the difficulties in trying to untangle freedom of thought from other rights in these chapters, the suggested answer is that the jury is still out.

The second side to the book’s examination of the right to freedom of thought is through a jurisdictional lens. The chapters on the specific countries – carefully curated to ensure that Asia, Europe, Africa and the Americas are represented – demonstrate why the right to freedom of thought may have languished in the shadow of more prominent rights, such as the right to expression. These chapters serve as standalone discussions, giving the reader an introduction to individual countries’ legal cultures and approaches to human rights protection. They also demonstrate how there are wide differences in the understanding of the right to freedom of thought, its place in constitutional law and how it is interpreted in the context of legislators and courts’ response to new technologies. Given that in some countries there is little pertinent case law or even domestic interpretation of the right, some chapters seem to discuss human rights in a generalised manner, losing some of the narrow focus suggested by the name of the Handbook. It could be that in their desire for broad geographical representation, the editors were willing to sacrifice some of the sharpness of their doctrinal lens. The result is a less tight or coherent middle section. Notable exceptions are the chapter on Colombia by Diego González Medina and Sebastián Rubiano-Groot Gómez with an in-depth discussion on case law related to the Constitution; and Eduardo A. Chia and Flavio Quezada’s chapter on Chile’s constitutional amendment to prevent assaults on freedom of thought by the unregulated adoption of advancing neurotechnologies.

Returning to my opening remarks, it is perhaps surprising that so little of this Handbook deals with neurotechnologies. The editors clearly made a choice when they concentrated the discussion on neurotechnologies in a single chapter. In it, Timo Istace and Catherine Van de Heyning give us a glimpse of a new world where commercial neurostimulation devices are now being marketed as being able to enhance cognitive capacities. Seemingly with scant scientific evidence, the manufacturers claim that these products will help users increase their attention, focus and memory. However, the marketing material fails to address concerns regarding the proprietary uses of neural data, the unreliability of inferences based on electrical impulses or potential life-changing consequences once these products are adopted in law enforcement and education. It is suggested that the right of freedom of thought can act as a bulwark against the potential of these technologies infringing on mental autonomy and mental privacy (p. 385). Should that strategy be successful, it will certainly help give the right the free-standing status it deserves.

In conclusion, this Handbook has managed both to fill a gap in the literature with admirably geographical breadth and at the same time demonstrated how there is a critical need to address the uncertainties regarding the scope of the right; a need that is becoming urgent with increasingly mentally invasive technologies. It is a must read for anyone who wants a thorough introduction to the right and how it is viewed around the world.