On What Matters is a two-volume book of moral philosophy by Derek Parfit published in 2011. It is a follow-up to Parfit's 1984 book Reasons and Persons. The dust jacket states that the book is "about reasons, values and morality". The book is introduced by Samuel Scheffler.On What Matters defends an objective ethical theory and suggests that we have reasons to act that cannot be accounted for by subjective ethical theories.