The only symbolic logic used in this book is a small part of propositional logic, also called sentential logic or Boolean logic. In this appendix, I review the relevant part of this simple area of logic and clarify some notation and terminology. This appendix is not an introduction to logic; various important fine points and distinctions are not be mentioned. But I hope this will suffice as an introduction to the basic ideas in the elementary part of logic used in this book, and as a clarification of the logical terminology and symbols used in this book.

The basic entities of the formal propositional calculus are usually called the propositions and the propositional connectives (and the language of propositional logic usually includes punctuation marks, usually parentheses, that are used to avoid ambiguity of grouping when “propositions” are “connected” in complex ways).

In this book, it is factors (or properties, or types) that play the role of the so-called propositions of propositional logic. The abstract and formal propositional calculus can be interpreted as applying to propositions in a number of ways in which the term “proposition” could be understood. For example, we could think of propositions as sentences (which may be understood as concrete linguistic entities such as utterances or inscriptions). Or we could think of them as statements (understood in such a way that many sentences can all be used to “make” the same statement, and the same sentence, if used in different contexts, would make different statements).