Lucretius (Titus Lucretius Carus) was a Roman poet and the author of the philosophical epic De Rerum Natura (On the Nature of the Universe), a comprehensive exposition of the Epicurean world-view. Very little is known of the poet's life, though a sense of his character and personality emerges vividly from his poem. The stress and tumult of his times stands in the background of his work and partly explains his personal attraction and commitment to Epicureanism, with its elevation of intellectual pleasure and tranquility of mind and its dim view of the world of social strife and political violence. His epic is presented in six books and undertakes a full and completely naturalistic explanation of the physical origin, structure, and destiny of the universe. Included in this presentation are theories of the atomic structure of matter and the emergence and evolution of life forms – ideas that would eventually form a crucial foundation and background for the development of western science. In addition to his literary and scientific influence, Lucretius has been a major source of inspiration for a wide range of modern philosophers, including Gassendi, Bergson, Spencer, Whitehead, and Teilhard de Chardin.
The Epicurean system consists of three linked components: Physics, Ethics, and Canonic. These three elements are designed to be interdependent, each one supposedly uniting with and reinforcing the other two. (To cite just one example, Epicurus' physics supposedly validates both the existence of free will and the fact that the soul disintegrates with the body, ideas that are crucial to Epicurean ethics. The canonic claims to validate the authority and reliability of sensation, which in turn serves as a basis for Epicurean physical theories and ethical views relating to pleasure and pain.) In actual fact, however, the three components are quite separable, and it is certainly possible, for example, to accept Epicurus' ethical doctrines while entirely denying his canonic teachings and physics.
One of the great achievements of the scientific imagination, the Epicurean cosmos is based on three fundamental principles: materialism, mechanism, and atomism. According to Epicurus the universe covers an infinitude of space and consists entirely of matter and void. For the most part the philosopher upholds Democritus' theory that all matter is composed of imperishable atoms, tiny indivisible particles that can neither be created or destroyed. He also shares Democritus' view that the atoms are infinite in number and homogenous in substance, while differing in shape and size. However, whereas Democritus held that the number of atomic sizes and shapes is infinite, Epicurus argued that their number, while large, is nevertheless finite. (As Lucretius notes, if atoms could be any size, some would be visible, and possibly even immense.) As for atomic motion, Democritus had claimed that the atoms move in straight lines in all directions and always in accordance with the iron laws of "necessity" (anangke). Epicurus, on the other hand, contends that their natural motion is to travel straight downwards at a uniform high velocity. At random and unpredictable moments, moreover, they deviate ever so slightly from their regular course, their resulting collisions thus occurring not by strict necessity but always with some element of chance. This theory of atomic "swerve" or clinamen is a crucial feature of the Epicurean world-view, providing (so Lucretius and other adherents believed) a firm physical foundation supporting the existence of free will.
Armed with these basic principles, Epicurus is able to explain the universe as an ongoing cosmic event – a never-ending binding and unbinding of atoms resulting in the gradual emergence of entire new worlds and the gradual disintegration of old ones. Our world, our bodies, our minds are but atoms in motion. They did not occur because of some purpose or final cause. Nor were they created by some god for our special use and benefit. They simply happened, more or less randomly and entirely naturally, through the effective operation of immutable and eternal physical laws.
Here it should be noted that Epicurus is a materialist, not an atheist. Although he argues that not only our earth and all its life forms, but also all human civilizations and arts came into being and evolved without any aid or sponsorship from the gods, he does not deny their existence. He merely denies that they have any knowledge of or interest in human affairs. They live on immune to destruction in their perfectly compounded material bodies in the serene and cloudless spaces between the worlds (intermundia), perfectly oblivious of human anxieties and cares. Lucretius imagines that Epicurus rivaled them in their divine tranquility.