These notes are based on a series of seminar lectures given during the 1951 Spring term at the Institute for Advanced study. Owing to limitations of time only particular topics were considered, and there is no claim to completeness. Since it is intended to publish later a more complete treatment of. the subject, comments about these notes as well as suggcstions concerning the desirabili ty of adding related topics will be appreciated and should be addressed to the author at the Hebrew University, Jerusalem, Israel.