This book examines the nature, sources, and implications of fallacies in philosophical reasoning. In doing so, it illustrates and evaluates various historical instances of this phenomenon. There is widespread interest in the practice and products of philosophizing, yet the important issue of fallacious reasoning in these matters has been effectively untouched. Nicholas Rescher fills this gap by presenting a systematic account of the principal ways in which philosophizing can go astray. Nicholas Rescher is Distinguished University Professor of Philosophy at the University of Pittsburgh, USA. In a productive research career extending over six decades, he has well over one hundred books to his credit, including Ethics Matters (2021), Philosophical Clarifications (2019) and Value Reasoning (2017).