The book is in three parts:
Part I. The Framework
Part II. The Terminology
Part III. Functions of the Common Sense
The stage is set by contrasting Plato's and Aristotle's views of the senses. In Plato's view the five senses are separate and the intellect integrates them. Aristotle's view of the matter tries to account for perception by nonhuman animals as well. So he says sensory input is integrated by the sensory capacity, by what he calls the `common sense', which also monitors sensory input. This monitoring function anticipates some modern conceptions of consciousness.
Aristotle conceptually divides the `soul' into different capacities. These are the nutritive, sensitive (perceptual and locomotive), and thinking. Plants have only the first one and only humans have the last one. The sensitive capacity of the soul is not an aggregate of the individual senses, but a unified whole. Memory, mainly in the form of images retained from sensory input, is also part of the sensitive capacity.
Integration recognizes the `common perceptibles', which are those perceived by more than one sense - change, rest, shape, magnitude, number, and one (or unity). These are sensed by both sight and touch. Part of the integration is cross-modal, e.g. that something is both hot and sweet, or colored and extended. The `special perceptibles' are those perceived by only one sense, such as taste, odor, and sound.
In sleep the common sense is incapacitated. Waking activates it. Aristotle argues that this is so because the common sense controls the peripheral sense organs. Awareness of an individual sense's activity or inactivity is the work of the common sense.
Four distinct functions of the common sense are identified: simultaneous perception, perceptual discrimination, control of the senses, and monitoring of the senses.
An interesting historical point is where Plato and Aristotle believed sensory integration occurred. Plato thought it was the brain, where he believed the rational soul was (in his dialogue Timaeus). Aristotle located the common sense in the heart. At the time there had been an intense and long standing debate among physicians regarding which organ it was. Plato sided with one school of physicians and Aristotle with the other.