Natural philosophy is the orderly investigation of the phenomena of the universe through the use of experimental observation and/or logical reasoning. It was introduce by Alexander the Great in his creation of the Library of Alexandria. This gave consent to the pursuit of knowledge and, since Alexander was taught by a natural philosopher, to the study of natural philosophy. In Ancient Greece (480-323 BCE), natural philosophers observed particular things like the phenomena of the natural world, along with abstract things like the nature of reality. These natural philosophers looked for universal principles by which phenomena could be explained, which among these philosophers included Heraclitus, Parmenides, and Plato. The philosopher, Heraclitus (ca. 545-475 BCE) of Ephesus, believed that flux, or change, was the basic principle of the universe. Parmenides (ca. 515-440 BCE), Heraclitus" successor, believed that Being is rational, that only what can be thought can exist. Since "nothing" cannot be thought, without thinking of it as something, there is no nothing, there is only Being. Plato (427-347 BCE), who was one of the most powerful thinkers in history, emphasized the immortal and unchallengeable consciousness over the mortal and changeful body. But Plato advanced a new division, favoring the invisible world of Forms, or Ideas, in opposition to the physical world. Evidently, Heraclitus, Parmenides, and Plato used the basic principle of change to explain the nature of the universe. .
As interpreted by the later Greek philosophical tradition, Heraclitus stands primarily for the radical thesis that everything is in flux. Born in 545 BCE in Ephesus, a city on the coast of modern Turkey, he was a man of strong and independent philosophical spirit. Unlike the Milesian philosophers whose subject was the material beginning of the world, Heraclitus focused instead on the internal rhythm of nature which moves and regulates things, namely, the Logos (Rule).