This chapter reprints four seminal essays written between 2000 and 2004. These essays explain why the familiar right to freedom of thought must be reconceived in an era of neurotechnology, such as human–computer interfacing, brain scanning, and “neuro-therapy.” To meet this challenge, these essays introduce the concept "cognitive liberty" and sketch the contours of a fundamental right by the same name. The first essay (in Part I) provides a brief overview of the right to cognitive liberty and why it is more challenging to define than bodily liberty. The second (in Part II) explains how the “war on drugs”—like censorship of books—has been a “war on mental states,” which, like book banning, implicates First Amendment principles. The third (in Part III) explains how Americans’ constitutional right to privacy should also encompass the privacy of our interior mental states. The fourth (in Part IV) revisits John Stuart Mill’s essay on liberty and explains why its logic applies to cognitive liberty.