Most religious thinkers accept duties independent of God's will. Rejecting DCT, they don't think that hatred is wrong because God forbids it (or even because a wise and loving God forbids it); instead, God forbids hatred because he knows that it's already wrong (and wrong in itself). While ethics and religion connect in many ways, they're also somewhat independent; so ethics doesn't presume religion or belief in God.

These next three chapters explore how this might work, using some ideas from natural law. This chapter (rationality) is about practical reason. The next (biology) is about our biological nature and what norms this leads to using practical reason. The third (spirituality) is about our relationship to God.

Natural Law

The term “natural law” (NL) refers both to moral principles, understood in a certain way, and to an ethical tradition, whose central focus is St. Thomas Aquinas (1224–74).

(1) “Natural law” refers to objective moral principles that are “written on the human heart” (as opposed to coming from society or revelation). Such norms are instinctive or based on ordinary reasoning. They're the same for everyone, authoritative over our actions, and known by virtually everyone.

Aquinas saw NL as part of how God uses laws to govern the world. He defined “law” as an “ordinance of reason for the common good, made by him who has care over the community, and promulgated.” He distinguished several types of law:

1. • Eternal law includes physical laws, moral laws, and revealed religious laws.

2. • Natural law (moral law) is that part of the eternal law that can be known by our natural reason and that applies to our free-will choices.

3. • Biblical law is law revealed through the Bible, to supplement and reinforce NL and to guide us to our supernatural goal of eternal happiness with God.

4. • Human law is civil law created by human societies to apply NL to particular circumstances; human rules that violate NL are considered, not unjust laws, but rather not laws at all.

Aquinas's central theme is the harmony between human reason and Christian faith. His ethics has two levels: moral philosophy is based on human reason and moral theology builds on this but also incorporates faith.