Descartes' "Meditations" are perhaps the classic work of philosophy. They address fundamental questions: What kinds of objects exist in the world? What kind of thing am I? Am I free? What is truth? What status do logical truths or mathematical theorems have? What can I know? Gregor Betz's systematic commentary reconstructs the corresponding lines of thought and justifications, attempting to provide answers to Descartes' questions. Other philosophers, particularly from the 20th century, are also engaged in a dialogue with Descartes. For the author of the volume, understanding a philosophical position ultimately means grasping what further theses it implies and which other positions it conflicts with.