Every day, in many situations, we use expressions which seem only vaguely to provide us with information. The weather forecaster tells us that "some showers are likely in Northern regions during the night", a statement which is vague with respect to number of showers, location, and time. Yet such messages are informative, and often it is not possible for the producer of the message to be more precise. A tutor tells his students that "only a few students fail their exams (...) outright". This does not give a precise incidence. Yet it might be equally misleading to do so. For example, to say that 12% failed outright last year says nothing about other years, while to say an average of 8% over the last five years says nothing about variability. We argue that a precise, numerical statement can be sometimes more misleading in reality than a vague statement. Many researchers in psychology have attempted to capture the meaning of quantities by relating them to scales of quantity. The book explores this idea in detail and shows with original studies how these expressions also serve to control attention and to convey information about the expectations held by those involved in the communication. The book works towards a psychological theory of the meaning of quantifiers and similarly vague terms. New links are drawn between formal theories of quantification and psychological experimentation. (shrink)