The U.S. Constitution is silent about a right to privacy, yet the Supreme Court determined in the 1965 reproductive rights case Griswold v. Connecticut, that such a right is inferred from the Bill of Rights. The reasoning in the landmark case hinged on an unexpected word—penumbra. The Court concluded, in other words, that the unmentioned right to privacy resides in the shadows of the amendments.

  But this was not the first time penumbral imagery was used in legal writings, and its broader cultural usage dates back much further. Jared Schroeder's The Penumbra Problem provides a much-needed—and quite colorful—social and cultural history of the term. The penumbra concept, it turns out, wasn't an accidental choice but instead a well-selected term with centuries of transnational history.

  Schroeder tells its story through two intertwining narratives: first, how the word penumbra transcended its oblique scientific origins to become a distinct cultural concept; and, second, the parallel story of its evolving use in legal thought. The book tracks back and forth in time, creating an unexpected lineage of people and works who became unwitting champions of the penumbra concept, including mathematician Johannes Kepler, feminist activist Estelle Griswold, British thinker Robert Hooke, and lexicographer Noah Webster. Pairing these stories with the writings of great legal thinkers such as Willam Douglas and Oliver Wendell Holmes, Schroeder is able to construct a kind of etymological love letter to this obscure concept.

  Not everyone has supported the privacy precedent, though, and it is no exaggeration to say that it is in danger today. Schroeder's clever, utterly unique history is, at its core, a call to redouble efforts to save the right to privacy. He argues that we must understand its ponderous, indeed uncertain, origins to fully appreciate the precarity in which it exists today, and only then can we adequately fight for it.