This book grew out of an international symposium, organized in September 1986 by the Austrian Cultural Institute in Warsaw in cooperation with the Polish Philosophical Society. The topic was: The Vienna Circle and the Lvov-Warsaw School. Since the two phil- osophical trends existed in roughly the same time and were close- ly related, it was one of the purposes of the symposium to investigate both similarities and thp differences. Some thirty people took part in the symposium, nearly twenty contributions were presented and extensively discussed. The sym- posium owed much to the excellent organization and warm hospital- ity shown by Dr Georg Jankovic, the Director of the Austrian In- stitute. As the person in charge of the scientific programme of the symposium, I take pleasure to ackwledge this debt. It so happened that a month later ather symposium of a similar character was held. It took place in the University of Manchester, on the occasion of the centenary of the births of Stanislaw Lesniewski, Tadeusz Kotarbiflski and Wladyslaw Tatarkie- wicz. Some papers read at the Manchester symposium form a part of the present volume. It was t possible, for technical reasons (the time factor was one of them), to include in this book all the material from the two symposia. Certain contributions have appeared elsewhere (for instance, K. Szaniawski's 'Ajdukiewicz on Non-Deductive Inference' was published in Danish Yearbook of Philosophy, Vol. 23). On the other hand, certain papers have been written special- ly for this volume.