Why it’s here. Carl Sagan wrote this book in the last years of his life, aware that he was dying, and it reads that way — not mournfully, but with the urgency of someone who has something genuinely important to say and is running out of time to say it. The book is a defense of science not as a collection of facts but as a way of thinking: skeptical, humble, self-correcting, and capable of producing genuine knowledge about the world. Sagan was worried — in 1995, when the book was published — about the rise of pseudoscience, the weakness of scientific education, and the willingness of populations to accept unfounded claims. He would find the current landscape more troubling still. That the book remains urgently relevant is not a compliment to our times.

What it offers. The center of the book is what Sagan called the “baloney detection kit” — a set of cognitive tools for evaluating claims, recognizing logical fallacies, and distinguishing genuine argument from persuasion dressed up as argument. These tools are not exotic; they are the ordinary equipment of careful thinking, described with unusual clarity and illustrated with examples ranging from UFO abduction claims to the Salem witch trials to cold fusion. Sagan is generous and respectful toward people who believe false things, understanding that the failure to think clearly is not a moral failing but a consequence of how human minds work and how little most people are taught to compensate for their natural tendencies. The book is also a meditation on wonder — Sagan’s case for science includes the case that science, properly understood, does not diminish the mystery and beauty of the natural world but deepens it. The chapters on skepticism and on the demands of evidence are the best brief treatment of the subject available to a general reader.

A word of caution. Sagan had an expansive and sometimes imprecise notion of what “science” covers. The book occasionally treats science as a sovereign method applicable to all questions, including some that sit at the edge of or outside its proper domain — social questions, ethical questions, questions of value. This is a mild but genuine overreach, and a careful reader will notice it. Sagan was also, understandably, more interested in the errors of believers in pseudoscience than in the sometimes parallel errors of scientific establishments; the book is better at tracking motivated reasoning in outsiders than in credentialed insiders. These are minor complaints about an important book by a man of great integrity. Read it, and then read further.