On our natural law moral realism, our duties flow from our nature, using practical reason. Humans have a triple nature – rational, biological, and spiritual:

1. 1. Rational: As rational beings, we're subject to norms of practical reason. In making decisions, we need to be vividly aware of the facts, avoid falsehoods, and be consistent (golden rule [GR], ends-means, reflective equilibrium, etc.).

2. 2. Biological: As rational members of the Homo sapiens species, we have additional duties.

3. 2. Spiritual: As rational creatures called to eternal life with God, we have additional duties.

This chapter focuses on our biological nature. We'll look for biological facts that lead, using practical reason, to general norms about how we, as human beings, ought to live.

On both divine command theory (DCT) and natural law (NL), our biological nature is from God and our duties are based on God's desires and purposes. But the views explain this differently:

1. • DCT: God creates humans in a certain way, because he has certain purposes for us. He then desires that we act in certain ways. Our duties come directly from God's desires.

2. • NL: God creates humans in a certain way, because he has certain purposes for us. Our duties come from our God-given nature using practical reason.

On NL, our duties come from how God created us.

Biological Human Nature

I was camping at remote Boucher Creek in the Grand Canyon, pondering a big question: “What is human nature? What is it to belong to the Homo sapiens species?” As I was thinking deeply, a squirrel came to my campsite, sniffing for food. I imagined the squirrel answering my question.

Squirrel Boucher began, “Judging from backpackers who camp here, humans are strange creatures. They're so unsuited to nature that they can survive only by taking a great number of things with them, which they load on their backs. It's comical to see these pitiful creatures stumble along on the trail, using only two legs, carrying so much weight! They bring food, water, clothes, shoes, shelter, tools, and so forth. They need such things to survive. Squirrels don't need any of this. If you drop a naked squirrel into a random wilderness without gear, it'll likely survive well. If you drop a naked human into a random wilderness without gear, it'll likely die.