in the poem operates according to these physical principles, guided by fortuna, "chance", and not the divine intervention of the traditional Roman deities and the religious explanations of the natural world. Within this work, Lucretius makes reference to the cultural and technological development of humans in his use of available materials, tools, and weapons through prehistory to Lucretius's own time. He specifies the earliest weapons as hands, nails, and teeth. These were followed by stones, branches, and, once humans could kindle and control it, fire. He then refers to "tough iron" and copper in that order, but goes on to say that copper was the primary means of tilling the soil and the basis of weaponry until, "by slow degrees", the iron sword became predominant (it still was in his day) and "the bronze sickle fell into disrepute" as iron ploughs were introduced. He had earlier envisaged a pre-technological, pre-literary kind of human whose life was lived "in the fashion of wild beasts roaming at large". From this beginning, he theorised, there followed the development in turn of crude huts, use and kindling of fire, clothing, language, family, and city-states. He believed that smelting of metal, and perhaps too, the firing of pottery, was discovered by accident: for example, the result of a forest fire. He does specify, however, that the use of copper followed the use of stones and branches and preceded the use of iron. Lucretius seems to equate copper with bronze, an alloy of copper and tin that has much greater resilience than copper; both copper and bronze were superseded by iron during his millennium (1000 BC to 1 BC). He may have considered bronze to be a stronger variety of copper and not necessarily a wholly individual material. Lucretius is believed to be the first to put forward a theory of the successive uses of first wood and stone, then copper and bronze, and finally iron. Although his theory lay dormant for many centuries, it was revived in the nineteenth century and he has been credited with originating the concept of the three-age system that was formalised from 1834 by C. J. Thomsen. Reception In a letter by Cicero to his brother Quintus in February 54 BC, Cicero said: "The poems of Lucretius are as you write: they exhibit many flashes of genius, and yet show great mastership." In the work of another author in late