Book excerpt: Logic is traditionally defined as a rule-based instrument, the application of which keeps the mind from making mistakes in reasoning. Logic teaches us various types of arguments and explains how to distinguish valid arguments from non-valid ones. This book is an English translation of Sayyid Ali Murtada’s 'Ilm al-Mantiq published in 2014. The Arabic original is based on Muhammad Rida al-Muzaffar’s (1904–1963) al-Mantiq, a textbook on logic used in Islamic seminaries. The present work is an introduction to logic and deals with topics ranging from conception, judgment, and signification to the four relations, the five universals, and the definition of propositions, contradiction, and syllogisms.