3.1. The Triune Psyche
Democritus, as well as Hippocrates (in his Corpus), concurred with Alcmaeon’s discovery of the brain as the seat of the mind (
6, 12). Alcmaeon of Croton ( Figure 1, left) has been called the real father of Greek medicine; he was the first to recognize the brain, or encephalos, as the central organ of consciousness and thought, or hegemonikon ( 2, 13). Democritus adopted and expanded on Alcmaeon’s view of the functions of the brain, as did Anaxagoras of Clazomenae and Diogenes of Apollonia, and developed a version that became especially influential because of its impact on Plato.
The psyche (mind, soul, vital principle) is made up of lighter, spherical, fast-moving atoms. Psychic atoms are dispersed among other atoms throughout the body and are especially numerous in the brain, which is the central organ of consciousness and thought. Life and psyche are movements of a subtle matter, made of spherical atoms, invisible corpuscles that span the entire body as mental principles (archae noerae). The function of sensation and thought (they are one) results from the mobility of the psychic atoms. Democritus and the other natural philosophers made no distinction between the mind (nous) and the soul (psyche); they are absolutely the same thing (
Atoms cruder than those of the mind are concentrated in the heart, making it the center of emotion; even cruder atoms are concentrated in the liver, which is the seat of lust and appetite (
14). Democritus anticipated Plato’s tripartite model of the psyche and named the brain the guard of the mind (phylax dianoies), the heart the queen, nurse of anger (vasilis orges tithenos), and the liver the cause of desire (epithymies aetion). Such a trichotomy developed into Plato’s hierarchy of the parts of the soul, set out in detail in Timaeus. Plato further developed his tripartite concept of the psyche in Phaedrus and in the Republic, whereby the organic seat of the rational or intelligent (logistikon), the temperamental or courageous (thymoeides), and the passionate or appetitive (epithymetikon), respectively occupy cerebral, thoracic, and abdominal portions of the cerebrospinal marrow ( 15, 16). In Galen’s subsequent theorizing, those three parts became the three pneumata of humoral physiology ( 3).
A fragment from Democritus’ book On human nature preserves the doctrines of the anatomical and functional localization of intelligence and the sentiments and an anatomical notion of the meninges and the sutures of the cranial bones: The brain guards the citadel of the body, ensuring its safety via coexisting nervous membranes (hymenes neurodeis); over those membranes doubled bones cover the brain, the guardian of the mind (
Democritus did not acknowledge the immortality of the psyche, and denied immortal gods and Pythagorean mysticism. After death, the psyche disintegrates into atoms; it perishes, in contrast with the Platonic view of immortality (
17) or even with Alcmaeon’s view that the soul is immortal as it is similar to the immortal objects by moving forever, because it and all the other objects move continuously (the sun, the moon, the stars, and heaven) ( 18). 3.2. Perception and Sensory Physiology
In humankind’s endeavor to understand ourselves, the long history of the development of ideas on perception extends to the time of Democritus. He had postulated that eidola, small images of external objects, were transmitted by atomic movements to the sense organs and from these to sensation. Such images bear an uncanny resemblance to what neurophysiologists call representations (
Democritus conceived of perception as a purely physical and mechanistic process; mind and senses were attributes of matter put together in a sufficiently fine and complex manner (
5). The perception of quality by the senses results merely from a specific quantitative distribution of atoms; this is the guiding principle in today’s neurochemistry.
Empedocles and Democritus expressed observations on sensory perception and adapted them to their general physical and philosophical theory. The mechanisms of sensory perception were further elaborated by Democritus. Perception happens only if the receptive organ has a geometrical shape similar to that of the particles approaching it from the object. Accordingly, sensory specificity is reduced to a structural geometrical specificity of the receiving organ (
Democritus specified that the emanation from the object formed a kind of imprint in the air which would be the immediate object of human perception (
6). He termed such an imprint, which shapes the space between object and eye, an eidolon, which means a representation, or Gestalt.
In addition, Democritus theorized that all the senses were forms of touch, which was the primary sense (
1, 2, 15) and made all things into passions of the senses ( 20). The number of sensations is greater than those perceived by the senses, and thus remain largely latent ( 1). 3.2.1. Vision
Regarding vision, the first intromission theories, in other words, the idea that vision involves something entering the eye from the object seen, were those of Democritus and Epicurus. These atomists believed that isomorphic images streamed off the objects and entered the eye, where they were sensed (
3). Expanding on the ideas of Empedocles of Acragas, Democritus suggested, in his now lost books (On mind, perception, and colors and On the diverse rhythms of atoms and the concepts of forms) that objects emit images (eidola, the actual term of Democritus being deikela), copies of their figure and color, which compress the air between the object of sight and the visual organ, and become imprinted and reflected in the moist eyes. Remnants of that theory can even be found in the writings of Descartes ( 6, 17).
More specifically, Democritus explained vision as the mirroring (emphasis) of objects in the water contained in the organ of vision proper (
15). Such a visual image in the pupil results from effluences (aporrhoae) both from the seen object and from the observer; the two meet and form a solid impression (entyposis) in the air, which then enters the pupil. Opposite of the centripetal emanation, from the object of vision to the eye, another emanation is conceived, centrifugal, simultaneous with or preceding the act of vision, from the eye to the object. Such a suggestion was in all likelihood made by Empedocles, and it was embraced by Democritus, Plato, and the Stoic philosophers. It is still used allegorically today in the expression: to throw a glance ( 6).
Democritus is the earliest philosopher in whose writings we find an attempt at a detailed theory of color vision. According to him, color is a purely subjective thing, in contrast with Plato, who regards colors as objective, and Aristotle, for whom color depends on both the eye and the object and the realization of the potentialities of the two (
15). Democritus considered color qualities related to the hypothetical structures of atoms. The theory of color perception arose from experiments with painters’ dyes and the optical effect of color mixtures ( 6). The basic colors were four, differing in atomic structure and corresponding to the four elements ( 17). Those color primaries were black (charred ivory or bone), white (powdered marble or pulverized seashells), red (vermilion), and yellow (earth ochre). Mixtures of the primaries can produce all known hues. The origin of the Democritean theory of vision may have been the observation of structural differences, such as smooth contours, brittleness and fragility of dyestuffs, in the quest for an objective basis of color perception ( 6). 3.2.2. Audition, Taste and Olfaction
The other senses are explained with emphasis on the different effects of the different sizes and shapes of atoms. We see, hear, smell, taste, and touch by the agency of varying atoms (
15). Sound is transferred when the particles of voice or noise mingle with similar particles in the air and thus, presumably, also form eidola; the air is broken up into bodies of like shapes and rolled along together with the fragments of the voice ( 3).
Individual tastes depend on the physical properties, the form of atoms. All sensible qualities rest on their order, shape and position (
15). Democritus appears to be the first scholar to propose a stereochemical theory for the chemical senses: sweet taste is produced by large, round atoms; sour by large, rough, angular atoms; acidic by sharp, angular, curving-thin atoms; pungent by spherical, thin, angular curving atoms; salty by large, not rounded, sometimes jagged atoms; bitter by small, spherical, smooth, of sinuous circumference atoms; and succulent by thin, small spherical atoms ( 17). Olfaction is also reduced to a tactile mode, like all the other senses. 3.2.3. Pain
On pain, Democritus reasoned that shed images, emanated by objects, may stimulate various receptors of the body and affect the mind. He viewed the intrusion of irregularly shaped, hooked atoms into the blood vessels as the cause of pain, experienced in the specific part of the body which is stimulated (