The first book to present Bruno de Finetti's theory of probability and logic of uncertainty in a systematic format. The author identifies de Finetti's "fundamental theorem of coherent prevision" as the unifying structure of probabilistic logic, highlighting the judgment of exchangeability rather than causal independence as the key probabilistic component of statistical inference. Throughout the text, readers are invited to address the subjective personalistic meaning of probability as motivating the mathematical construction. Philosophical attention to meaning is shown to support a difference approach to statistical practice that is widely prevalent today. Includes numerous examples, problems, and illustrations to facilitate understanding.