This essay will show that Locke’s teaching on the law of nature is not based on divine revelation, or a juridical doctrine of individual rights, or self-ownership, or self-preservation, or reasoning from premises that are not rooted in the empirical world. I will argue, on the contrary, that the real ground is found in his understanding of the conditions of human happiness. This conclusion is far from evident on the surface of Locke’s writings. Locke draws his reader into an amazingly complex line of reasoning, scattered up and down in several of his books, leading finally to the real basis of his teaching on the law of nature. Locke engages the reader in a dialogue, in which initially plausible arguments are put forward, then implicitly questioned, leading to new arguments, which again are questioned, and so on. Locke says that “long and sometimes intricate deductions of reason” are necessary to discover the law of nature. Locke writes treatises, not Platonic dialogues. Nevertheless, a dialogical thread will take us from one of Locke’s books to another, until we put together all the relevant passages to show the complete picture of his argument.