Probability is indispensable to modern science, yet its meaning remains one of the most contested questions in philosophy. Does a probability describe an objective feature of the world, a long-run frequency, a logical relation between evidence and hypothesis, a causal disposition, or a rational degree of belief?

This book offers a historically informed and systematic introduction to the principal interpretations of probability. It examines the classical, frequentist, propensity, logical and subjective Bayesian traditions, together with contemporary best-system accounts of objective chance. Rather than treating these views as isolated schools, the authors compare them through explicit criteria: their treatment of single cases, reference classes, initial probabilities, evidential updating, objectivity and decision under uncertainty.

The concluding chapters develop a criterion-guided pluralism and apply it to quantum physics, statistical mechanics, population genetics, epidemiology, economics and the social sciences. Accessible without sacrificing conceptual rigour, the book provides students and researchers with a clear map of what is at stake whenever probability is used to describe the world, assess evidence or guide rational action.