Beginning with Xenophanes, the Presocratics attempted to define their experience of the world in terms of nature, itself. Rather than imagining that there were supernatural beings driving the natural world, the Presocratics searched for the forces of nature within nature. They challenged the traditionally held religious ideas by asking what exactly is the one thing that the world comes from. This is the concept of "first principles". The inquiry into the single source of nature was based on the belief the Presocratics held that knowledge is not the result of revelation, but is available to anyone who looks to their perceptions of the world around them. Xenophanes was the first to say that knowledge is acquired through discovery. The idea that knowledge comes from discovery allowed t Xenophanes, Tales, Anaximander, Anaximenes, and Heraclitus to each look to one single element or essence to understand the universe. The importance of "first principles" lies in the reliance on nature rather than the supernatural. Each philosopher chose his own principle; this concept did not lead to the assumption of one universal principle. Heraclitus believed that the first principle was fire; Tales chose water, Anaximenes air, and Anaximander the Unlimited. Each one questioned how his chosen principle could exhibit itself in the natural world that is available to human discovery. This provided more of a challenge because the order of things could not be attributed to the will of a being with thoughts, emotions, desires, and behaviors similar to those of human beings. Heraclitus, for example, chose fire to symbolize his idea that everything in the world i

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