The opening of Leviathan is devoted to outlining the mechanics of the human mind and to explaining the phenomena of sense perception, understanding, and processes of thought. Hobbes bases all his claims in his materialist conception of the universe as a plenum, filled with matter. All natural activity results from material bodies moving and colliding with one another, transferring motion from one body to another. All our sense perceptions result from material bodies colliding with our eyes, ears, nose, mouth, or body. The motions of the bodies with which we collide set off a series of motions between our sensory organs and our brain. Any body, once set in motion, will remain in motion until acted upon by another body.

Imagination results from continuing motion within the brain after the initial sensation of the foreign body causing that motion has passed. Understanding is a certain kind of imagination relating to the sense of signs and words. Memory is another kind of imagination, relating to the continuing motion set off by a sensation in the past. The motion of imagination continues until it is met by some hindrance, and in certain complex scenarios, it can build upon collisions with other motions to produce trains of thought. These trains of thought can be either unguided, as in dreams, or regulated, when the thinker intentionally directs his or her mental activity in a particular direction.

Hobbes moves on to discuss the various applications of regulated, or directed, thought. Language, reason, and science are chief among these applications. Language, or speech, was invented for the purpose of putting “mental discourse” into “verbal discourse.” The transformation of the mental into the verbal allows us to name the conclusions reached by certain trains of thought without having to reconstruct the train of thoughts constantly and allows us to communicate mental discourse to other people. Speech has four principle uses: (1) to record knowledge of things, (2) to communicate this knowledge to others, (3) to communicate intentions and desires to others and ask for help, and (4) to entertain ourselves by playing with words. Speech can also be abused, and the chief abuses of language include the use of lies, the use of metaphorical language, the shifting of meanings between words, and the use of language to injure other people.

Speech is composed primarily of names, or appellations, and the connections between them. Truth and falsity are categories that apply only to speech and do not exist outside of speech. The precise meanings we ascribe to different words must be consistent and commonly accepted for us to be able to recognize truth. Once common definitions are determined, true conclusions can be made by building logically on previous definitions. These accepted terms, and the truths they represent, are called first principles and are the necessary bases for meaningful philosophical discourse.

Our sensory experience of the world is not objective but is instead always influenced by our own subjective characteristics—physical, emotional, prejudicial. As long as differences remain in the approaches diverse individuals take toward reality, certain agreement regarding the meanings of words is impossible. It is impossible to simply look to nature itself as a basis of truth. Rather, there must exist in human society some central authority to decide the singular definitions of all words and to determine first principles. Although the faculty of reason allows us to apprehend the laws of nature, we do not all reason in exactly the same way. To maintain a peaceful and functional linguistic system of meaning—and a peaceful society—humans must agree to uphold the reasoned dictates of one central authority. Hobbes elaborates a complete program, based on reason, for modifying philosophical and scientific inquiry to provide secure knowledge and put an end to disagreement and social discord. The reasoning process, built on agreed-upon first principles, must produce steps that are complete and provable on their own. Each step must be made by a mathematical manipulation of terms, adding and subtracting apt names, describing connections between assertions, and determining consequential relationships among natural phenomena.

Hobbes elaborates on his description of the natural universe as a mechanical system in which motion is transferred from body to body. In living animals, two sorts of motion exist: vital and voluntary. Vital motions are those motions that take place unconsciously and that support life, including such basic bodily functions as breathing, circulating blood, and digesting food. Voluntary motions are invoked by active decisions, including moving limbs, speaking, and walking. The physical causes that precipitate these voluntary motions are the motions of thoughts and imagination. Hobbes calls these thoughts “endeavor.” Endeavor can be broken down further into “appetite” (or “desire”) and “aversion.” Human nature essentially consists of the interplay between appetite and aversion. All humans are possessed of a great many appetites and aversions, including those we are born with that aid our survival and those we acquire from experience.