No doubt every reader of this journal is aware that computer science is becoming infiltrated by a strange breed of people called logicians, who try to convince computer people that their arcane symbolism and obscure terminology are just what is needed to solve the software crisis, the hardware crisis, and any other difficulties that the computer world finds itself facing. Unfortunately the symbolism and the jargon can be very off-putting to anyone who has not already become immersed informal logic; I have often met people who work with computers and are aware of how important logic is claimed to be by its devotees, and who feel that they really ought one day to make an effort to penetrate its mysteries, but who have not known how to set about doing so. This article and its sequel (‘Logic as a Formal Method’) are intended as a fairly gentle initiation into what logic is about and what it has to offer computer scientists. They are inevitably very sketchy and incomplete – more like the brochures that can be picked up at a travel agent's than a proper guide-book – but it is to be hoped that some, at least, of my readers will come away with a clearer picture of what lies in store for them if they decide to follow up the more detailed references. The present article outlines the two systems which form the standard core of formal logic, the propositional calculus and the predicate calculus. For a more detailed treatment, see my good Logic for Information Technology (Wiley, 1990). The second article looks at applications of these systems to computer science, and then expands the horizons by looking at some of the non-standard logical systems that have developed from the standard core and which have found application in a computational context.