In Ionia of the 6th century B.C. the economic, social and political condidions which allowed the birth of philosophy were combined for the first time. Miletus, the metropolis of many colonies, received various original incentives, which led to a new perception of the world and the problems that are related to it. The first three philosophers were citizens of Miletus and were known as natural philisophers because the object of their philosophy was the natural world. Thales (624-547 B.C.), Anaximander (610-546 B.C.) and Anaximenes (585-525 B.C.) explicitly denied the mythological and religious interpretation of the world and tried to explain its origin in a materialistic way, based that is, on an original substance, the motion and the changes of which create every object and phenomenon. The three thinkers answered the ontological question of philosophy with materialistic criteria, but they did not raise at all the question of the theory of knowledge, since they considered the possibility of knowledge of the world obvious and unquestionable. Each one was systematically engaged in sciences, such as mathematics and astronomy, and established the basis of the Greek exact sciences. Nothing has survived from their philosophical work and all we know about it comes only from references and criticisms of posterior philosophers.

Thales seems that he was familiar with the achievements of the Egyptians and Chaldaeans, but his philosophy was never completely detached from the practical problems of his time. He considered water a primary element, in which the world floats. By that he followed a tradition already expressed in Homer in a mythological way. If we accept the information about Thales, from a poet of the 5th century B.C., then he was the first to support the immortality of the soul, a fact that must be connected with his sojourn in Egypt. Although very few things from those referred to Thales must be considered real, many particular discoveries are attributed to him, such as the solstices and their circles, the five celestial zones, the source of lunar light, the cycle of the floods of the Nile and others.

Anaximander was considered Thales' student and it seems that he travelled a lot in his lifetime, and that he played a leading part in the establishment of the Milesian colony of Apollonia on the Black Sea. He considered the Infinite to be the first principle of the world that is usually interpreted not as a material mixture of everything but as an inexhaustible initial cause. It is said that his scientific achievements include the first map and the conception of the spherical model of the sky, where the earth with the shape of a cylinder held the central position. He developed rationalistic observational thought and, irrespective of the factual mistakes, he helped to develop mathematics and astronomy. Pythagoras followed the same path. On the other hand, his doctrines about the fiery nature of stars and the succession of worlds preannounce the cosmic deity of Xenophanes.

Anaximenes, the student of Anaximander, believed air to be the primary substance of the world. According to his theory, the alternation of natural forms and conditions was owed, to heating or cooling and the condensation or dilution of air. In his cosmological pattern the earth was flat and shallow, whereas the firmament was a kind of transparent membrane, onto which the stars were nailed.

Heraclitus, another Ionian philosopher, supported, developed and enriched these materialistic principles of the natural philosophy with dialectic elements.