To update the view of human nature that undergirds eighteenth-century British/American political economy, this article reviews literature from diverse subfields of psychobiology. Findings on the structure, function, and evolution of the human brain confirm the duality between reason and passion that is at the core of the science of Hobbes. Contemporary findings across fields indicate that people become emotionally attached to objects, including verbal abstractions, through experiences with pleasure and pain. In contrast, human reasoning is essentially scientific. The duality between passionate motivation and humanity's unique capacity for reasoning makes political science important. By applying the scientific method to the subject of politics, people can design institutions that channel quasi-rational behavior toward outcomes that are mutually beneficial, rather than mutually destructive. Defining human nature correctly is the key to political science, and Smith's addition of the passion of sympathy to Hobbes's narrow definition of human motivation is essential.