We wish to give a precise formulation of the intuitive concept: The degree to which the known facts support a given hypothesis.

Includes chapters on scientific language, mathematics, probability, credibility and induction, scientific explanations, life, and science and values.

In 1948 Hempel and Oppenheim proposed an explicatum for the concept of systematic power 1, pp. 164–167. Since that time some shortcomings have been found in this first attempt. It is the purpose of this paper to show that one can keep the basic approach of the ‘48 paper, and overcome the known disadvantages by means of changes in the details of the definition. In this improvement certain tools will be used that were not available in 1948.

Reading of this complex subject-matter is considerably simplified by a variety of devices: Brief summaries at the beginning of chapters and sections; theorems, definitions, etc. are clearly marked for easy cross-reference, the most important ones being specially marked by "+" signs. For a brief survey, the reader can read the summaries and the marked theorems and definitions.

Although the work consists of sixteen connected chapters, there is a difference of motivation that led to the book being published in two volumes. The first eleven chapters, comprising Volume I, attempt to teach good guessing through examples. The remaining chapters, in Volume II, deal with general principles of plausible reasoning.