On the Nature of Things (De natura rerum)
On the Nature of Things is the usual English title of a short Latin treatise on the physical universe by Isidore, bishop of Seville, composed in Visigothic Spain and dedicated to King Sisebut. (It should not be confused with the didactic poem of the same English title by Lucretius, whose De rerum natura is an entirely different work.) Isidore’s book is a compact handbook of cosmology, astronomy, meteorology, geography, and time-reckoning, arranged in a sequence of some forty-eight brief chapters. Its importance lies in its design: it is the first work of natural science by a Christian author that is not a commentary on the six days of creation in Genesis. Instead of expounding Scripture verse by verse, Isidore takes over the classical description of the physical cosmos and fills it with an eclectic mixture of pagan and patristic learning, adding Christian reflection and allegory at selected points. He also supplied the book with circular diagrams (rotae), from which it acquired the medieval name Liber rotarum, “the Book of Wheels.”
Design and occasion
The work was written at the request of Sisebut, who had put questions to Isidore about celestial phenomena. Isidore’s answer is not a specialist’s treatise but an elementary and orderly survey, meant to give clergy and students a reliable account of what the created world is like and how its parts fit together. The method is compilatory: he excerpts, condenses, and harmonizes a range of authorities — Latin poets and encyclopedists, cosmological writers, and Christian authors — frequently explaining the name of a thing as a key to its nature, in the manner familiar from his better-known Etymologies. The result is an essentially Greco-Roman picture of the universe, framed throughout by the conviction that the cosmos is a created, ordered whole whose structure and numbers are intelligible and meaningful.
The heavens
The treatise begins with time-words — the day and the night — before moving outward to the heavens. Isidore describes the heaven itself, its spherical form, and the revolution of the firmament; then the sun, its size, heat, and course; the moon, its light, phases, and circuit; and the eclipses of both. He treats eclipses as natural events with regular causes rather than as portents produced by magic, and explains the interposition of the earth or moon that produces them. From there he turns to the stars: the fixed stars and their ordering, the constellations and the zodiac, the five planets with their courses and relative positions, and comets, whose fiery, transient character he discusses with care. Much of this material derives from classical astronomical lore, but Isidore regularly folds in a moral or theological observation — the heavens as the declaration of divine craftsmanship, the harmony of the celestial motions, the smallness of the earth by comparison with the great bodies above it.
Air and weather
The middle of the book descends into the atmosphere. Isidore treats the air, clouds, rain, thunder, lightning, the rainbow, hail, snow, ice, and mist, and gives an extended account of the winds: their number, names, directions, and qualities. His procedure is characteristic — a brief statement of what a phenomenon is, followed by an explanation of its name and a summary of received opinions on its cause. Some explanations are drawn from Greek physics, some from Latin compilers, and some are frankly theological, presenting weather as the operation of hidden natural causes ordained by the Creator. Alongside the physics runs a steady interest in the marvels of the natural world, the kind of material that made the book so useful to medieval readers.
The waters and the earth
The later chapters concern the globe and its parts. Isidore presents the earth as a sphere and discusses its position in the cosmos, the division of the world into regions, and the great encircling ocean with its gulfs and seas. Rivers, springs, lakes, and the causes of their movement receive attention, as do mountains, the shaking of the earth in earthquakes, and islands. The treatment is geographical as well as physical: the known regions of the world, their names, and their characteristic features are catalogued alongside explanations of how water flows, why springs rise, and what causes the earth to tremble.
The reckoning of time
The book closes with the measurement of time, the subject that gave it much of its later usefulness. Isidore treats the hours, the day, the night, the week, the months, the solstices and equinoxes, the seasons, and the year, and discusses the various ways different peoples have counted and named these units. He explains the divisions of time in terms of the motions of sun and moon, and he carefully distinguishes the natural intervals of the heavens from the conventional reckonings established by human custom. This section, and the chapters on celestial motion that precede it, made the work a standard resource for computists — those who calculated the calendar and the date of Easter — in Ireland, Anglo-Saxon England, and Frankish Gaul.
The final chapters turn from the measurement of time to its meaning. Isidore sets out the ages of the world, aligning the six days of creation with the successive epochs of sacred history and ending with the present age. The cosmological survey thus concludes as a scheme of salvation history: the physical cosmos, ordered in number and measure, is the setting in which human history unfolds and comes to its end.
Text, diagrams, and later form
The work survives in several recensions — a short, a medium, and a longer form — which differ in the number and arrangement of chapters and in the presence of added material; the fullest form includes chapters not found in the shorter ones. Text and image were conceived together: the circular diagrams that accompany particular chapters are not ornament but part of the exposition, presenting the circuits of the heavens, the divisions of the year, and other cyclical patterns in visual form. Isidore’s book was recast by Bede in his own earliest work of the same title, which removed the chapters on the units of time to a separate treatise on times, and it was quarried throughout the Middle Ages by computists, cosmographers, exegetes, and encyclopedists, and repeatedly imitated under the same title by later writers such as Hrabanus Maurus and Alexander Neckam.
Atoms, void, and natural explanation
The poem sets out a physical system in which everything is compounded of imperishable atoms moving in void. Coming-to-be and passing-away are explained as the temporary combination and dispersal of these constituents, not as the creation or destruction of matter itself. This physics is put to polemical use: natural phenomena are to be explained by atomic motion and the conditions of bodies, not by divine manufacture or arbitrary intervention. The book repeatedly connects this explanatory project with a therapeutic aim — freeing the mind from fear by showing that the world operates through natural causes.
Soul, death, and the fear of the gods
A central strand of the argument concerns the soul. It is treated as corporeal and mortal, composed of fine atoms, so that death is the dispersal of sensation rather than the beginning of conscious punishment. This account is directed against the fear of death and the religious anxieties that, in the poem’s view, distort human life. The gods, if they exist, are conceived as tranquil beings remote from human affairs, not as agents of reward and punishment; false beliefs about them are presented as a source of disturbance rather than consolation.
Cosmos, perception, and human culture
The poem surveys the heavens, the earth, and living things within the same naturalistic framework. Worlds form and perish; species and societies emerge through trial and adaptation; and human civilization develops from rude beginnings through discoveries such as fire, language, agriculture, law, and the arts. It also treats perception and error, explaining how the senses and the mind can be misled and how false beliefs about the gods and death arise and persist.
Poetry and persuasion
The work is a didactic poem, using verse and vivid imagery to make a philosophical system persuasive and memorable. Its significance lies in the sustained attempt to unite physics, psychology, ethics, and a critique of religion in a single Latin poem, and in its influence as a transmission of Epicurean thought and as a model for later didactic and natural-philosophical poetry.
