The language of Computational Logic used in this book is an informal and simplified form of Symbolic Logic. Until now, it has also been somewhat vague and imprecise. This additional chapter is intended to specify the language more precisely. It does not affect the mainstream of the book, and the reader can either leave it out altogether, or come back to it later.

In all varieties of logic, the basic building block is the atomic formula or atom for short. In the same way that an atom in physics can be viewed as a collection of electrons held together by a nucleus, atoms in logic are collections of terms, like “train”, “ driver” and “station”, held together by predicate symbols, like “in” or “stop”. Predicate symbols are like verbs in English, and terms are like nouns or noun phrases.