This course is primarily intended to serve as an introduction to classical informal logic. Since its inception in the fourth century B.C. logic has been regarded as a branch of philosophy studying the principles of correct reasoning as they are manifested in various uses of natural language. Admittedly, under the influence of mathematical procedures the leading conception of logic has dramatically changed in the nineteenth and twentieth centuries. The advances in formalization and computability made a significant impact on the character of logical study as well, so much so that many philosophers now view logic rather as an independent discipline, very close or even identical with mathematics. As a result, after Frege and Russell logic started to be equated with formal or symbolic logic. The only open question about its nature seemed to have become whether it is a branch of mathematics or rather its foundation. In both cases, however, it was assumed the study of logic should devise solid formal rules of deduction that guarantee absolute exactness, necessity and certainty.