His work on nature is known through citations of his books on the subjects, "On the Nature of Man", "On Flesh" (two books), "On Mind, On the Senses", "On Flavors", "On Colors", "Causes concerned with Seeds and Plants and Fruits", and "Causes concerned with Animals" (three books). He spent much of his life experimenting with and examining plants and minerals, and wrote at length on many scientific topics. Democritus thought that the first humans lived an anarchic and animal sort of life, going out to forage individually and living off the most palatable herbs and the fruit which grew wild on the trees. They were driven together into societies for fear of wild animals, he said. He believed that these early people had no language, but that they gradually began to articulate their expressions, establishing symbols for every sort of object, and in this manner came to understand each other. He says that the earliest men lived laboriously, having none of the utilities of life; clothing, houses, fire, domestication, and farming were unknown to them. Democritus presents the early period of mankind as one of learning by trial and error, and says that each step slowly led to more discoveries; they took refuge in the caves in winter, stored fruits that could be preserved, and through reason and keenness of mind came to build upon each new idea.