Two Essays on Entropy by Rudolf Carnap (edited with an introduction by Abner Shimony) brings a major twentieth-century philosopher of science to the front lines of thermodynamics, probability, and inductive logic. Written during Carnap’s 1952–54 fellowship at the Institute for Advanced Study, these essays pursue an “abstract concept of entropy” usable for scientific inference while testing, with unusual clarity, the coherence of the physicists’ own statistical notion. Carnap’s guiding claim is bold and bracing: entropy belongs with temperature and pressure as an objective physical magnitude, not a merely logical or informational index. From this stance he scrutinizes the classical formulations of Boltzmann and Gibbs, rejects fashionable identifications of entropy with “negative information,” and articulates a principle of physical magnitudes to require that finer descriptions of a system accord with coarser, thermodynamic attributions. Shimony’s editorial introduction situates Carnap’s project within postwar debates on probability (frequency, propensity, and epistemic readings), ergodicity, and coarse- versus fine-grained ensembles, clarifying both the reach and the limits of Carnap’s proposal. Presented together for the first time as Carnap had originally envisioned, the essays are lightly but thoughtfully edited: overlapping prefatory sections are removed, a concise “Brief Formulation” is foregrounded, and cross-references rationalized to reveal the architecture of the program as a whole. Readers see Carnap extend Boltzmann’s entropy beyond cell partitions, probe the logical pitfalls of description-dependent definitions, and sketch a continuous, geometry-based alternative aimed at eliminating arbitrary coarse-graining. The result is a rare conversation across philosophy and physics—historically grounded, methodologically incisive, and still sharply relevant to contemporary work in statistical mechanics, information theory, and the foundations of data-driven inference. A vital resource for scholars in philosophy of science, physics, and the history of analytic philosophy, Two Essays on Entropy restores a rigorous, physicalist account of order, randomness, and explanation to center stage. This title is part of UC Press's Voices Revived program, which commemorates University of California Press’s mission to seek out and cultivate the brightest minds and give them voice, reach, and impact. Drawing on a backlist dating to 1893, Voices Revived makes high-quality, peer-reviewed scholarship accessible once again using print-on-demand technology. This title was originally published in 1977.