Often described as his life's work, Fleming worked on this non-fiction book for over thirty years. It was completed just before his death and published posthumously on the 7th of July 2011.
Many reviewers have found it hard to categorise, with John Thackara describing it as "half encyclopedia, half commonplace book, half a secular bible, half survival guide, half ... yes, that’s a lot of halves, but ... I have never encountered a book that is so hard to characterise yet so hard, despite its weight, to put down ... It’s an incredibly nourishing cultural and scientific treasure trove."
Lean Logic explores themes including ethics, science, relationships, culture, policy, art and history, but unconventionally for a book of such varied themes, it is structured in dictionary format, with each entry followed by a list of other related entries. This allows Fleming to highlight connections that might otherwise be overlooked without detracting from his in-depth exploration of each theme, and also has the effect of allowing the reader to follow the narrative of their choice as they explore Fleming's thoughts and research on strategies for the future.
His vision of the future is challenging, as he sees in the present "an economy that is destroying the very foundations on which it depends" (ecologically, economically and culturally), but many reviewers have commented on the positive spirit and humour that suffuse its pages as Fleming describes strategies and principles for a satisfying, culturally rich future in such difficult circumstances.