In order to give a clear account of contemporary methodological theories we require a terminology with precisely established meanings. For this reason it will be necessary to preface the discussion proper with some terminological definitions. The purpose of this is not to establish any theoretical principles but to obtain rules for using certain words and phrases; these rules will often take the form of propositions which could lso be used to make assertions about things: but here all that matters is the way in which the terms in question will be used in this book.