In the Preface to this enormous work, Derek Parfit makes a reference to Sidgwick's ‘great, drab book’ The Methods of Ethics (xxxiii). The juxtaposition of adjectives is amusing and oddly endearing; for Parfit, Sidgwick's drabness is not in any serious competition with his greatness. Parfit's two-volume book is not drab – though it can be exhausting – and, although it is probably too early to tell for sure, it also shows certain signs of greatness. Time and time again, one is impressed, even moved, by Parfit's enthralling and indefatigable resourcefulness and level of immersion in these issues, and by his dogged and transparently sincere commitment to discovering the truth about them. Despite its length, On What Matters demands careful study by everyone with a serious interest in philosophical ethics.