At the start, Hobbes maps out the structure of his book, which is divided into four parts. Part 1 defines humanity and the natural laws by which it is governed. Part 2 details the basis on which covenants between rulers and subjects are built in sovereign states—which Hobbes refers to as commonwealths—and how these commonwealths may be preserved or dissolved. Part 3 presents arguments pertaining to the relationship between civil power and ecclesiastical power in Christian commonwealths. Finally, Part 4 identifies and critiques a number of faulty philosophical and religious arguments that threaten the spiritual and intellectual health of man and the commonwealths in which he resides.

Hobbes begins his discussion of humanity with an investigation into sense and perception. Given his persistent view that the universe is governed solely by matter and motion, Hobbes characterizes perception as an external object colliding with our senses either directly, as in the cases of touch or taste, or via the function of nerves, as in sight. He contrasts this view with the teachings of contemporary universities that promote the Aristotelian argument that objects radiate an incorporeal apparition or aspect with which our senses interact.