Homer and Hesiod, along with other poets, centuries before the pre-Socratics, thought that true knowledge was exclusive to the divine. But starting with Xenophanes, the pre-Socratics moved towards a more secular approach to knowledge. The pre-Socratics sought a method to understand the cosmos, while being aware that there is a limit to human knowledge. While Pythagoras and Empedocles linked their self-proclaimed wisdom to their divinely inspired status, they tried to teach or urge mortals to seek the truth about the natural realm—Pythagoras by means of mathematics and geometry and Empedocles by exposure to experiences. Xenophanes thought that human knowledge was merely an opinion that cannot be validated or proven to be true. According to Jonathan Warren, Xenophanes set the outline of the nature of knowledge. Later, Heraclitus and Parmenides stressed the capability of humans to understand how things stand in nature through direct observation, inquiry, and reflection. Theology Pre-Socratic thought contributed to the demythologization of the Greek popular religion. The narrative of their thought contributed to shifting the course of ancient Greek philosophy and religion away from the realm of divinity and even paved the way for teleological explanations. They attacked the traditional representations of gods that Homer and Hesiod had established and put Greek popular religion under scrutiny, initiating the schism between natural philosophy and theology. Pre-Socratic philosophers did not have atheistic beliefs, but it should be kept in mind that being an atheist those days was not without social or legal dangers. Despite that, arguments rejecting deities were not barred from the public sphere which can be seen in Protagoras's quotation on the gods: "About the gods I am able to know neither that they exist nor that they do not exist." The theological thought starts with the Milesian philosophers. It is evident in Anaximander's idea of the apeiron steering everything, which had other abilities usually attributed to Zeus. Later, Xenophanes developed a critique of the anthropomorphism of the gods. Xenophanes set three preconditions for God: he had to be all good, immortal and not resembling humans in appearance, which had a major impact on western religious thought. The theological thought of Heraclitus and Parmenides is not entirely certain, but it is generally accepted that they believed in some kind of divinity. The Pythagoreans and Empedocles believed in the transmigration of souls. Anaxagoras asserted that cosmic intelligence (nous) gives life to things. Diogenes of Appollonia expanded this line