Book I: Of Man
Chapter 1: Of Sense
Chapter 2: Of Imagination
Chapter 3: Of the Consequence or Trayne of Imaginations
The first three chapters of Leviathan concern the mechanics of the human mind, covering the topics of sense, imagination, and the train of thought. Hobbes argues that our knowledge of the world originates from "external bodies" pressing against our sensory apparatus. Envisioning the universe as a plenum constituted solely of matter, Hobbes depicts objects continually bumping against each other and describes the passage of motion from one material body to the next. This elementary motion of the universe eventually transfers to the surface of the human body, where nerves and membranes of the eyes, nose, ears, tongue, and skin are physically moved, in turn relaying their acquired motions on to the brain. "Sense," then, is the action of external bodies colliding with our sensitive organs.
Matter cannot move itself, Hobbes declares (in challenge to the philosophy of vitalism, which maintained that matter was self-motivated). Consequently, "when a thing is in motion, it will eternally be in motion" unless acted upon by another body. Hobbes deduces that this continuance of motion is responsible for the transformation of sense into thoughts or "imagination," for when an external body presses against the human sense apparatus and sets off a series of new motions, these motions will perpetuate until they meet a hindrance. The duration of sensory motion after the fact is called "decaying sense," which becomes Hobbes's definition of imagination. To illustrate, Hobbes suggests that the persistence of a vision after the eyes have been closed indicates that the ocular sensory apparatus is still in motion; this motion is no longer immediate sensation, but imagination. Such imagination, over time, is the same as "memory." Memory of things sensed from the outside world is defined as "experience," while sensation of internal movements of the human body is called a "dream" when one is asleep, or a "vision" or "apparition" when one is awake.
"Understanding" is a particular form of imagination, defined as the idea produced by the physical sensation of words or visible signs. A complex variety of understanding is the "train of thoughts" or "mental discourse," in which the succession of one imagination upon another, one internal sensation provoking the next one, initiates the process of thinking. There are two possible trains of thoughts: the "unguided" train, in which mental discourse wanders in no particular direction, as in dreams; and the "regulated" train, in which the thinker directs mental discourse in a specific direction. By tracing the transfer of motion from external matter to the human body, Hobbes has deduced a mechanism of the human mind—namely, the passage from sense to thought to train of thoughts—in which sensory experience of the world is funneled into regulated and directed thinking. Building upon this foundation, Hobbes next considers the logical developments of directed thought: language, reason, and science.
In the manner of a geometrical proof, Hobbes's philosophical method proceeds from one conclusion to the next in logical succession. As Leviathan consists of an interconnected series of propositions and ideas, the text appropriately begins with chapters examining the nature and origin of ideas themselves.
The rest of Hobbes's argument depends upon the conclusions established in these opening chapters. The propositions about human thought form the first principles for the geometrical proof that Hobbes is attempting to construct. Hobbes makes his arguments in a series of steps; the validity of the claim of each step is based upon the claim made in the previous step. However, the very first principle on which Hobbes bases his claims regarding the nature of thinking--namely, that the universe is a plenum filled completely with material bodies--is never articulated in the text.
Hobbes's assertion of a plenum is his response to a years-long philosophical debate against vacuism, or the theory that the universe is largely devoid of matter. Still, though Hobbes claims (as we will see in the next section) that philosophical truth must be deduced from shared definitions, he does not here indicate that his own fundamental first principle of the plenum is generally accepted or agreed upon; Hobbes acts as his own arbitrator and judge of first principles. His philosophical project manages to remain logically consistent only by recursively validating these first principles in later chapters. To dispute the truth value of Hobbes's unspoken claim that nature is a plenum is not necessarily to dispute the entire edifice that is Leviathan, for Hobbes argues from common experience at several points. However, so tightly structured is the text, with one step leading to the next step, with one layer founding the succeeding layer, that—as with a house of cards—tearing out the bottom tier would threaten to topple the upper stories.
Of course, as we will see in the next section, Hobbes is proposing an epistemological system whose foundations need not be universally true as long as they are conventionally agreed upon for the sake of attaining civil peace. This factor alone prevented Hobbes's vacuist contemporaries from dismissing his project on the basis of its controversial first principles.