Leviathan is short on physiological detail as to the inner-workings of the mind, but Hobbes' ideas provide the conceptual background for the major schools of thinking about thinking which follow in the next two centuries -particularly the skeptics and associationists. The text presents itself as a mediating step between Burton and Locke. It expands and sharpens many of Burton's ideas about categories of thought and, as Locke will later do, bases a total socio-political philosophy upon them.
Hobbes' model is based upon an application of the rules of geometry and physics to the human sciences. Hobbes professed, first and foremost, a theory of motion. Following the theories of Galileo (whom he visited in Europe) he believed that, contrary to the opinion of his day, all matter was in motion and would remain in motion unless acted upon by another force. Based upon this philosophy, Hobbes constructs a model of the human psyche in which all thought is explained by the motion of things in the material world impacting the senses, which creates a subsequent motion in the senses, which creates a subsequent motion in the brain, which continues to exert its pressure on the brain until its motion is degraded sufficiently by the interference of other new motions. Three fundamental premises underlie this model: 1) that everything is material, including the mind, and the soul (689-693); 2) that we are brought into the world with the mind a tabula rasa (85); and 3) that the senses are responsible for all mental activity (85-87). Based upon these primary tenets, Hobbes constructs an elaborate model of categories of thought which build one upon the next until one reaches the highest levels of abstract thought. First, there is the Representation or Appearance which is the initial motion carried by the senses to the brain (85). Once a representation enters the brain, it follows a Trayne which is the course of its motion in the brain as it interacts with other representations. Newer representation will, necessarily obscure older representations in the brain; hence, as a representation proceeds along its Trayne, its influence becomes lesser. As Hobbes puts it, the sense 'decays'. The interaction of these Traynes of varying degrees of magnitude, Hobbes terms Imagination, which is, he claims, "nothing more than decaying sense" (88). It is, however, "the first internal beginning of all Voluntary Motion" (118). [Note: Hobbes is very explicit about the fact that Imagination defines the particular state of all the various Traynes that are present in the mind at one time and not the process of decaying, which he terms Memory (89).]
Having defined the imagination, Hobbes goes on to refine his definition by distinguishing between two distinct types: Simple and Compound. Simple imagination describes the presence of a particular Trayne separate from all others, "as when one imagineth a man, or horse, which he has seen before (89), and Compound imagination describes the interaction of separate Traynes, "as when from the sight of a man at one time, and of a horse at another, we conceive in our mind a Centaure" (89).Individual Traynes are subject to two types of development within the brain: Regulated and Unguided (95). Unguided thoughts are those "wherein there is no passionate thought, to govern and direct those that follow, to itself, as the end and scope of some desire, or other passion: In which case the thoughts are said to wander, and seem impertinent one to another, as in a Dream" (95). Regulated thoughts, to the contrary, are those that are directed by some "desire, and designe" such as fear (95). (Where this fear comes from he neglects to convincingly explain.) At this point in his model, Hobbes makes an important leap to the realm of language. Expanding upon an earlier definition of the progress of the imagination as "mental discourse" (94), Hobbes claimes that the function of speech is to transfer our mental discourse to verbal discourse (101). From this definition, Hobbes then constructs of model of Understanding as "nothing else, but conception caused by speech" (109), marking not a separate function of cognition, but rather a particular group of Traynes. -those initiated by the exposure of the senses to speech. The last area of cognition which Hobbes defines is that of the Reason . According to Hobbes, "When a man Reasoneth, he does nothing else but conceive a summe total, from addition of parcels; or conceive a remainder, from Subtraction of one summe from another: which, if it be done by Words, is conceiving of the consequence of the names of all the parts, to the name of the whole; or from the names of the whole and one part, to the name of the other part" (110).
Having dealt with what he perceived to be all the categories of cognition, Hobbes goes on to attempt to explain the sources of Appetites or Desires and the Will. His explanation begins with a type of stimulus response which he calls a Voluntary motion (118). The Voluntary Motions are the result of the senses affecting the inner organs of man. They are a type of pre-wired response within the organs to particular stimuli. Different stimuli cause different types of Voluntary Motions, known as Endeavours (119); and when an Endeavour becomes directed at an external object, it becomes a Desire or an Aversion (119). The Will, subsequently, is the last appetite or aversion in which mental motion gets converted into physical motion (127).
Several interesting and important observations and theories arise as Hobbes explains his model of cognition and attempts to extrapolate a theory of politics from it. Of particular interest is the way in which Hobbes deals with the materiality of the human subject. As noted earlier, Hobbes states specifically that all things, including thoughts, are material; however, his model of cognition still predicates a strange type of division between the individual thinking subject and the rest of the material world; for, according to Hobbes we never actually experience the true materiality of the thing we sense. "The cause of Sense, is the external body, or object, which preseth the organ proper to each sense, ...which pressure, by mediation of the Nerves, and other stings, and membranes of the body, continued inwards to the Brain and Heart, causeth there a resistance, or counter-pressure or endeavour to the heart, to deliver it self: which endeavour because Outward, seemeth to be some matter without. And this seeming, of fancy, is that which men call Sense ...But their appearance to us is Fancy, the same waking that dreaming. And as pressing, rubbing, or striking the Eye, makes us fancy a light ...the object is one thing, the image or fancy is another" (85). Here we see the seeds of later Skeptical thought which argued similarly that all experience is really perception and that we have know real knowledge of the material world. In Hobbes, this philosophy becomes particularly interesting because insists, in the face of this skepticism, in maintaining that all thought is still material.
It should also be noted that Hobbes both inherits and proliferates many of the standard divisions of mental function from his period -Namely, Fancy, Imagination -including a subsequent two part sub-division, Reason, Understanding and Will- although his use of these terms is quite different than many other mainstream authors. Also of interest to scholars of Cultural Studies and literature in particular is Hobbes' treatment of language. Hobbes devotes an entire chapter to language and its right usage, during which he espouses an interesting model of the function of language in political society (Chapter 4) -one in which metaphorical language is specifically derided.