Hobbes advances his reductionism succinctly: mental phenomena are ultimately and completely reducible to mechanical events among bodies in motion. "Thinking" he defines as a distributive process whereby the material constituents of a human brain are so ordered in space as to represent various (external) momenta under the traditional rubrics of "objects" and "qualities."
From the vantage provided by his reductionism, Hobbes proceeds to consider the origins of intellection: ideas derive from images, images from sensations, sensations from the motions of external bodies as communicated to the sense organs. The sort of derivation Hobbes proposes here is not, however, semantic. Ideas are not, in other words, connected by truth-functional relations: they are connected by causal relations. Consequently, he insists that the explication of an idea is simply the tracing of its causal history--a history that inevitably terminates in a sensation. The analysis of sensation accordingly plays a central role in Hobbesian psychology because it is in sensation that all ideas ultimately originate.
The bulk of Chapter One is thus devoted to providing a mechanical account of sensory experience. Hobbes describes how sundry sequences of (external) physical motion irritate the human sensory surfaces, thereby initiating further sequences of (internal) motions, terminating in the application of "pressure" to the brain and heart. The heart in turn reacts by exerting a "counter-pressure" so as to resist the intruding motions. It is this resistance which, according to Hobbes, we denote by the general term 'sensation.'
What is of interest here is not Hobbes' primitive billiard-ball model of the afferent nervous system; it is rather his identification of sensation with the efferent activity of nervous impulses. It is precisely this identification that enables him to distinguish primary and secondary qualities in mechanical terms. The primary qualities (magnitude and momentum) individuate (real) external objects and characterize objective space. The secondary qualities (color, sound, odor, texture, etc.) individuate sensible objects and characterize phenomenal or sensuous space. It is in virtue of their possessing primary qualities that external objects irritate our sensory surfaces. The physiological mechanisms whereby the human nervous system resists irritation, however, account for why we fail to perceive the primary qualities directly, and instead project the secondary qualities onto external objects.
Hobbes concludes his analysis of perception with a sketch of the rival analyses then current in scholastic, neo-Aristotelian circles. Basically, he accuses the schoolmen of mysticism, that is, of offering an account of perception in terms of occult, formal causes rather than the pellucid, quantitative, mechanical processes adduced within his own system of psychology.