All material bodies are either elemental substances or compounded out of a union of these substances. The elemental substances are indestructible and indivisible. This is proved by the necessities of thought (i. 498, etc.) and of Nature. If there were no ultimate limit to the divisibility of these substances, if there were not something immutable underlying all phenomena, there could be no law or order in the world. The existence and ultimate constitution of the atoms is thus enunciated—
Sunt igitur solida primordia simplicitate
Quae minimis stipata cohaerent partibus arte.
Non ex illarum conventu conciliata,
Sed magis aeterna pollentia simplicitate,
Unde neque avelli quicquam neque deminui jam
Concedit natura reservans semina rebus.[380]
At this stage in the argument, from line 635 to 920 of Book I, the first principles of other philosophies, and particularly of the systems of Heraclitus, Empedocles, and Anaxagoras, are discussed at considerable length, and shown to be inconsistent with the actual appearance of things and with the principles already established.
A new departure is made at line 920, and it is shown that the atoms must be infinite in number, and space infinite in extent;—the contrary supposition being both inconceivable and incompatible with the origin, preservation, and renewal of all existing things. It is shown also that the existing order of things has not come into being through design, but by infinite experiments through infinite time. The doctrine that all things tend to a centre is denied, and the book concludes with the imaginative presentation of the thought that, if matter were not infinite, the whole visible fabric of the world would perish in a moment, 'and leave not a rack behind.'
The second book opens with an impressive passage, in which the security and charm of the contemplative life is contrasted with the restless anxieties and alarms of the life of worldly ambition. The argument then proceeds to explain the process by which these atoms, primordial, indestructible, and infinite in number, combine together in infinite space, so as to carry on the birth, growth, and decay of all things. While the sum of things always remains the same, there is constant change in all phenomena. This is explicable only on the supposition of the original elements being in eternal motion. The atoms are borne through space, either by their own weight, or by contact with one another, with a rapidity of motion far beyond that of any visible bodies. All motion is naturally in a downward direction and in parallel lines, but to account for the contact of the atoms with one another it must be supposed that in their movements they make a slight declension from the straight line at uncertain intervals. This liability to declension is the sole thing to break the chain of necessity—'quod fati foedera rumpat.' It is through this liability in the primal elements that volition in living beings becomes possible.