Logic, Language, Formalism, Informalism is about the process of formalisation in computing, language theory and mathematics, and is designed for computer science students studying a course in logic. The text sets out to explain proof and reasoning in a clear and accessible manner and is based on the premise that computer science students require an understanding of the reasoning systems that underpin much computing theory. In examining the process of formalisation, the book also criticizes and challenges the results, helping students to clearly understand these fundamental issues. The language Prolog is used to explain the concepts.