Thisisaslightyrevisedversionofthe1985editionofmylogicbook.Manyty-posanderrorshavebeencorrectedandthelinedrawingshavebeenimproved.Mostmistakeswereminor,exceptforasubtleerrorinTheorem4.3.3.Indeed,thesecondpartofthetheoremaboutthecomplexityoftheprooftreeTob-tainedfromaresoluti