The right to freedom of thought encapsulates the idea that certain parts of the mind should be categorically out of governmental reach. But both the scope of the right and the meaning of “thought” remain unresolved. At the example of privacy of thought, this chapter analyzes ten candidates for understanding the provision, engages with scholarly positions, addresses implications of the sparse case law, and draws on the classic internum-externum distinction. An interpretation of Article 18 of the International Civil and Political Covenant (ICCPR) according to the canons and some constructive considerations lead to the proposal of a robust scope, encompassing thoughts as mental entities and thinking as a mental activity. The chapter also proposes a test for interferences and sketches some implications, especially with respect to emerging technologies.