This book may be interesting and useful both for a philosophical reader who wants to learn some basics elements of category and topos theory, and for a mathematical reader interested in philosophical interpretations of these theories.

MathSciNet

[Badiou's] mathematics is precise and correct ... I am impressed by the lucidity of [his] remarks on the philosophical significance of category theory, especially in relation to set theory, and I invite philosophically minded mathematicians to be so too.

Notices of the AMS

Battered photocopies of Badiou's hand-drawn primer on category theory were prized possessions among the small group of people in Paris who gathered to attend his Saturday morning seminars in the mid-1990s, and coupled with the companion volume on <i>Being-There</i> also translated here, <i>Topos</i> remains a vital source of information for one of the most important and most challenging sequences of Badiou's philosophical trajectory. In addition to the distinctive light they shed on the transition from volume one to two of Badiou's <i>Being and Event</i>, both these texts are also of great interest and pedagogical value in their own right: non-specialists won't find a clearer, more accessible and more stimulating philosophical introduction to these crucial fields of contemporary mathematics.

Peter Hallward, Author of Badiou: A Subject to Truth and Professor of Philosophy, Kingston University, London, UK.