The ancient era is mainly comprised of the work of Greek and Roman thinkers. During the ancient era, Greek thinkers were the most innovative. The Romans' philosophy stemmed mostly from the Greeks. They built upon it, but they did not add many new theories. Greek philosophy is was composed by three groups: the pre-Socratics, Socrates and his students, and the thinkers that followed.
In the 6th century BC, the Greeks endeavored to unravel the secrets of the universe and life. These individuals were the first metaphysicians on earth, searching for the reality hidden behind appearances.
One of the first men who sought to explain the world without depending on mythology was Thales of Miletus. He was a mathematician who believed that nature (water in particular) was the origin of life. He based his theory of water as the fundamental building block of matter on his discovering of sea fossils. He was followed by two of his countrymen, Anaximander and Anaximenes. Anaximander theorized that the world originated as a series of conflicts between exact opposites, like hot and cold and wet and dry. Anaximenes thought that the atmosphere was the origin of substance. He is most notable, however, for first conceptualizing the laws of conservation of matter and energy. He asserted that nothing can be created from nothing. Matter, force, and energy are indestructible.
Pythagoras was another philosopher/mathematician. He believed that mathematics is the basis of reality because the relations of things can be illustrated numerically.
Heracletus put forth that the only constant in the universe is, paradoxically, change. Stability is only an appearance. For instance, the sun appears each day in the same pattern as it always has, so we assume that it is a fixed point. However, the sun is dragged tens of thousands of miles each day in orbit around the galaxy's center. So, although it appears constant to us, it is forever changing. Parmenides disagreeed with Heracletus. He said that permanence is real and change only an delusion.
All of these philosophers are called monists, because they sought their explanations in a single element of nature. Later philosophers combined theories for more complex explanations. Empedocles was the first to state that four basic elements exist: earth, wind, fire, and water.
Anaxagoras taught that matter is made of infinitely small particles. Democritus carried this idea further by naming these particles "atoms" and defining them more completely as tiny, indivisible bodies. He said that atoms were small, round, and solid.
The Sophists, the world's first skeptics, appeared in the late 5th century. They taught that philosophy should be used to survive in the practical world. Protagoras was the name of the first Sophist. He believed that the real world is the material world and saw the concept of worlds of the mind as senseless conjecture. He began among people a distrust of pure speculation and metaphysics.
The teachings of Socrates gave birth to the classical period of Greek philosophy, which lasted from about 430 to 320 BC. The classical Greek philosophers believed that anything humans could experience or think about was worth investigating. Without the insistence by the Greeks on thinking rationally about all subject matter, there may have been no social sciences today. But because those early scholars were philosophers, what their teachings remained part of philosophy for many centuries.
Socrates said it is conceivable to master pure virtue and achieve truth. He sought fundamental principles and first posed some of the basic questions of ethics. He did this by the Socratic method, a way of teaching and learning that is based on question-and-answer conversations. Socrates' philosophies were grounded by two basic assumptions: that a person is never to do wrong and that no one who knows what is right will act contrary to it.
Plato was Socrates' most outstanding student and recorded many of their conversations. He developed mulitple philosophies that include theories of knowledge, human conduct, politics, and the universe. He expressed the belief of a world of sensory experiences that constantly changes. He also believed in a world of constant thought, which is the only true reality.