The explosive progress of logic, since Frege, has produced applications in linguistics, mathematics and computer science. Students and practitioners of any of these fields, and of philosophy, will find this book an excellent reference or introduction. Now expanded to include non-classical logic, logic for the computer, and more. The central concepts are explained as they come into play in informal writing and conversation --argument, validity, relevance, and so on. This study guide progresses to concepts such as probability calculus. The aim of this book is to server as an introduction and reference text to students in all these related fields. The book presupposes no previous acquaintance with the subject and may be used as a text for an introductory course, as a problem supplement to other texts or as a guide for self-study.