One of the characterising theses of early analytic philosophy is that psychological questions should be distinguished from philosophical and logical questions. Psychology is therefore not understood as an important science for the origin of analytic philosophy. Those readers of Bertrand Russell and G.E. Moore who have stressed the importance of British idealism for the origins of analytic philosophy have shown that anti-psychologism was already an important feature in the writings of F.H. Bradley. Mathematics and a non-psychological treatment of mathematical problems have shaped the early development of Russell's philosophy towards analytic realism. These characteristics of early analytic philosophy do not imply, though, that psychology did not have any influence on the origins of analytic philosophy. The thesis of this book is that analytic philosophy can partly be understood in light of the development of psychology as an empirical science at the end of the nineteenth century. Not all kinds of psychology will be relevant for this thesis. Franz Brentano has made a distinction between descriptive and genetic psychology. Genetic psychology investigates the development of the mind and gives inductive laws; descriptive psychology is able to give exact laws, because it is concerned with a descriptive analysis of the mind.