They said that all things are formed by the coalescence of certain unchangeable, indestructible, and impenetrable particles which they named atoms; the total number of atoms is constant; not one of them can be destroyed, nor can one be created; when a substance ceases to exist and another is formed, the process is not a destruction of matter, it is a re-arrangement of atoms.
Only fragments of the writings of the founders of the atomic theory have come to us.

The views of these philosophers are preserved, and doubtless amplified and modified, in a Latin poem, _Concerning the Nature of Things_, written by Lucretius, who was born a century before the beginning of our era.

Let us consider the picture given in that poem of the material universe, and the method whereby the picture was produced.[2] [2] The quotations from Lucretius are taken from Munro's translation (4th Edition, 1886).
All knowledge, said Lucretius, is based on "the aspect and the law of nature." True knowledge can be obtained only by the use of the senses; there is no other method.

"From the senses first has proceeded the knowledge of the true, and the senses cannot be refuted.

Shall reason, founded on false sense, be able to contradict [the senses], wholly founded as it is on the senses?
And if they are not true, then all reason as well is rendered false." The first principle in nature is asserted by Lucretius to be that "Nothing is ever gotten out of nothing." "A thing never returns to nothing, but all things after disruption go back to the first bodies of matter." If there were not imperishable seeds of things, atoms, "first-beginnings of solid singleness," then, Lucretius urges, "infinite time gone by and lapse of days must have eaten up all things that are of mortal body." The first-beginnings, or atoms, of things were thought of by Lucretius as always moving; "there is no lowest point in the sum of the universe" where they can rest; they meet, clash, rebound, or sometimes join together into groups of atoms which move about as wholes.