This book is all about one simple idea: that speakers convey information not only by what they say, but also by what they don't say. In fact, the idea is so obvious that we may never know who had it first. What is known is that around the mid-19th century, John Stuart Mill thought it obvious enough to be mentioned almost in passing:

One century later, it was H. Paul Grice who saw that this simple idea contains the gist of a general framework for pragmatics, based on the premiss that discourse is a joint project undertaken by speakers who expect each other to be cooperative. It is this expectation, according to Grice, which gives rise to the pragmatic inferences he calls “conversational implicatures”. The inference Mill refers to is such an implicature; more specifically, it is a quantity implicature.

If it is so simple, why does it take a book to explain? There are several reasons.