The most private of all human experiences are unexpressed thoughts and feelings. Until they are translated into verbal expression or body language, such mental experiences are inaccessible to everyone but the person who is doing the thinking or the feeling. This has been apparent to the community of scientists who research our inner lives. “Dreams,” as one psychologist notes, “cannot be observed by anyone but the dreamer while they are happening” (Domhoff 2003, 39). Researchers can thus learn about a dream’s content only when the dreamer emerges from sleep, and tells what she can remember of her imagined travels. Those who study waking consciousness face a similar predicament: Although someone who is awake – unlike a sleeping subject – can describe her feelings and thoughts as they arise, this description is necessarily an incomplete translation. For better or worse, a person’s internal images are – as one recent study of consciousness states – “available directly only to the owner of the mind in which they occur” (Damasio 2010, 70). They are, in the words of another, confined to a “private theater” or sorts, a place where each self is not only the creator of an internal mental script but also its only audience (Edelman & Tononi 2010, 20).

What is a barrier to science, however, is a boon to privacy. The inherently private nature of our mental lives may sometimes stymie the efforts of psychologists to understand our minds, but it also shields our internal thoughts from other observers who we very much want to keep out. Indeed, it provides each person the only space in which a person can be sure that his secret plans, goals, or fantasies will remain secret. Locked drawers can be forced open. Electronic storage centers can be hacked. But our inner thoughts will, by their very nature, remain hidden until we reveal them.

This, in any case, is the state of affairs with which we are familiar – and which has provided the background for our existing privacy laws. It is not, however, a state of affairs that will necessarily survive the coming decades of the twenty-first century. There is because there are two important cracks in the protective wall that nature erects around our thought process, both of which might be dramatically widened by emerging technologies. While an outside observer cannot observe a person’s thoughts or feelings, that observer might draw inferences about our inner thoughts based on two kinds of phenomena that can be observed: (1) Observations or records of our external behavior and (2) The physical correlates or causes of mental states in biological activity in the brain.

The first of these is more familiar, to lawyers and lay people alike: While we could conceivably generate and develop an idea entirely inside of our own minds, that is usually not how we go about expanding our understanding of the world, or exercising our creative and intellectual powers. Rather, we draw upon – and create records of our thinking in – the external world. We purchase and read books, search for information on the World Wide Web, or engage in conversation and collaboration with others. Such reading choices and Internet activities are records of our thinking processes. In fact, they are often more than records: When a scientist scribbles down an equation on a notepad or a writer sets down story ideas in a journal, she is often not simply recording a thought fully generated in silent contemplation, but rather working to create that idea in the first place. The writing is an essential component of the thinking process, not simply a record of it (Clark, 2008, xxv).

Indeed, even when we are zealous enough about the privacy of our thoughts and plans to avoid writing them down – or revealing them in a series of Web searches – it is almost impossible to hide them entirely. We tend to reveal clues about our plans and interests, for example, in the stores and others locations we choose to visit, the people we choose to associate with, and the questions we ask of others as we seek information in our daily lives. Although it was, in the past, difficult for any one observer to piece all of these clues about our thinking, it becomes easier for government (as well as certain large corporations) to do so as more and and more of our interactions with the world are recorded – by the location tracking that occurs as we carry SmartPhones, by video cameras that record increasing amounts of what occurs public space, or in transaction data generated and stored each time we make a purchase, or have an event ticket, personal identification, or license plate scanned as we enter a monitored area. Courts applying constitutional law have already grappled with how to understand these technological threats to our mental privacy.

The other breach in the protection that nature provides for our mental privacy is less familiar to courts, and less pervasive and familiar a part of everyday life: With emerging forms of technology, scientists might infer our mental states from the biological activity that generates these thoughts. Consider what scientists have recently been able to figure out about what is occurring in individuals’ minds from scans of their brains. Using functional Magnetic Resonance Imaging (fMRI) to study brain activity, they have been able to tell what type of object a person is silently contemplating: One type of signature pattern of brain cell activation arises when someone thinks of a hammer, another when someone thinks of a house (Shinkareva et al. 2008). Brain scans have likewise revealed which room within a “virtual house” a person is navigating through (Hassabis et al., 2009). They have revealed whether a particular environment was new or familiar to the viewer (Smith 2013). They have revealed which clip in a series of film clips a person was remembering at a given time (Chadwick et al. 2010). And, in some of the most impressive demonstrations of this technology’s potential, they have allowed scientists to reconstruct – entirely from fMRI readings of the brain activity in an individual’s visual cortex – a rough reproduction of an image she was viewing or a video she was watching (Nishimoto et al. 2010). Brain scanning technology, in other words, has allowed scientists to take a set of perceptions from an individual’s “private theater” – and rescreen them for outside observers. Such a technology might one day let dream researchers watch a person’s dreams – or at least elements of them – on a video screen, instead of learning about them second-hand later from a subject’s best (but often somewhat unsuccessful efforts) to remember his imagined adventures and translate them into words. Indeed, such a primitive fMRI-based “dream recorder” was built by a team of researchers in Japan allowing them to determine (with 60% accuracy) which objects the subjects of the experiments had reported remembering from their dreams (Stromberg 2013).

The fMRI scans are not the only technology scientists have used to uncover the physical correlates in the brain of different cognitive tasks or feelings. Unlike fMRI technology, which can only be used when individuals lie in a massive cylindrical scanner inside a laboratory, functional near infrared imaging (fNIR) produces similar maps of brain activity, but does so by shining specific wavelengths of near infrared light into an individual’s cortex from a portable head set (Ayaz et al. 2011). Electroencephalography (or “EEG”) devices are also cheaper and more easily used than are fMRI machines: EEG has long been used to measure the rhythms of electrical activity (“brain waves”) that occur as neurons generate electrochemical signals throughout the brain (Marcuse et al. 2016, 1–10, 12). They typically generate such measurements from multiple electrodes placed over a person’s scalp (Marcuse et al. 2016, 1–10) but makers of video games and biofeedback devices have recently marketed headbands and helmets that allow individuals to transform brain waves into a form that can be viewed on a computer, or perhaps used as a system to control video game play (Childers 2013).

Certainly, at present, none of this technology – which I will refer to under the heading of “neuroimaging” – currently allows scientists, or anybody else, to engage in the kind of detailed mind reading one finds in science fiction and fantasy. Individuals cannot, as in Philip K. Dick’s story, Ubik, steal commercially valuable data from each other’s minds (Dick 2007). Existing neuroimaging is nothing like the magical “pensieve” device that characters in Harry Potter use to immerse themselves in each others’ vivid three-dimensional memories (Rowling 1998). Nor is it like the fictional brain tapping that the heroes of the movie, Inception (2010) and the villains of the television show, The Prisoner, use to enter and pry secrets from one another’s dreams (Carraze & Oswald 1996). The modern-day uses of neuroimaging – to divine mental content – focus on telling whether someone is lying or not, or whether someone recognizes an image or other stimulus. In such cases, it is the researcher who provides much of the content (in the form of a question to be answered, or a word or picture to react to), and the brain under observation then provides some indication of whether there is deception in the person’s behavior, or that what they are seeing is familiar. Even these modest forms of “brain-based mind reading” (to borrow a term used by Francis Shen) (Shen 2013, 710) are in early stages of development, and not yet admissible as evidence of dishonesty, for example, in US courts. Nor are they yet in use by US law enforcement. Of course, that may change as the technology develops – and the examples of fMRI experiments that reveal more about mental content, like a word or concept that someone is concentrating on, or the imagery they are looking at in their mind’s eye, provides hints of what more powerful neuroimaging may be able to do in the future.

My focus in this book is on the second of these two threats to our mental privacy: the possibility that neuroimaging will reveal unexpressed thoughts that we have succeeded in keeping out of Internet records, and hidden from government or other video cameras, or other surveillance technologies. How should courts, lawmakers, and legal scholars address this technology? First, how should they think about the threat to privacy it presents? Should they view it as a privacy concern only if and when it matures into something closer to the mind reading we currently find only in science fiction – something that might allow officials to look and listen into the private scenes and dialogues of unexpressed thoughts? Or should constitutional limits and other privacy laws block even the more limited and fragmentary glimpses that fMRI, fNIR and EEG technology might already provide into our mental operations? Do brain scans become a significant privacy problem only when they reveal substantially more about our thoughts and intentions than what may already be re...