On What Matters (3 volumes)

Derek Parfit's On What Matters represents one of the most significant contributions to moral philosophy in recent decades, though its relevance to the God debate emerges indirectly through its treatment of objective moral truth. This monumental three-volume work defends moral objectivism against various forms of relativism and subjectivism, arguing that there exist mind-independent moral facts that provide reasons for action regardless of individual desires or cultural contexts.

Parfit develops what he calls the "Triple Theory," attempting to reconcile three major ethical traditions—Kantian deontology, contractualism, and consequentialism—by showing they converge on the same fundamental principles when properly understood. This synthesis matters for theological discussions because it addresses whether morality requires divine grounding. Parfit explicitly argues that objective moral truths exist as necessary, non-natural facts that do not depend on God's existence or commands. He contends these normative truths are irreducibly normative, neither reducible to natural facts nor requiring supernatural explanation.

The work engages critically with moral nihilism and error theory, particularly addressing arguments from philosophers like John Mackie who deny objective moral facts. Parfit's response involves sophisticated meta-ethical arguments about the nature of reasons and normativity. He maintains that just as mathematical truths exist independently of human minds or divine decree, so too do moral truths. This position challenges both divine command theory and naturalistic reductions of morality.

Significantly, Parfit addresses the is-ought problem and argues for non-naturalist moral realism without invoking theological foundations. His approach suggests morality's objectivity neither requires nor precludes God's existence—moral facts simply exist as part of reality's fundamental structure. This stance positions him against both theistic ethics that ground morality in God and atheistic views that embrace moral skepticism or relativism.

The work's contribution to the God debate lies in demonstrating how robust moral objectivism might be maintained without theological commitments. Parfit shows that taking morality seriously—believing in genuine right and wrong, better and worse—does not necessitate religious belief. This challenges both theists who claim morality requires God and atheists who assume rejecting God means embracing moral relativism. His sophisticated defense of secular moral realism provides important philosophical resources for those seeking objective ethics without metaphysical theology.