It is the aim of the present study to introduce the reader to the ways of thinking of those contemporary philosophers who apply the tools of symbolic logic to classical philosophical problems. Unlike the "conti­ nental" reader for whom this work was originally written, the English­ speaking reader will be more familiar with most of the philosophers dis­ cussed in this book, and he will in general not be tempted to dismiss them indiscriminately as "positivists" and "nominalists". But the English version of this study may help to redress the balance in another respect. In view of the present emphasis on ordinary language and the wide­ spread tendency to leave the mathematical logicians alone with their technicalities, it seems not without merit to revive the interest in formal ontology and the construction of formal systems. A closer look at the historical account which will be given here, may convince the reader that there are several points in the historical develop­ ment whose consequences have not yet been fully assessed: I mention, e. g. , the shift from the traditional three-level semantics of sense and deno­ tation to the contemporary two-level semantics of representation; the relation of extensional structure and intensional content in the extensional systems of Wittgenstein and Carnap; the confusing changes in labelling the different kinds of analytic and apriori true sentences; etc. Among the philosophically interesting tools of symbolic logic Lesniewski's calculus of names deserves special attention.