The classical heritage

The early pre-socratians had not yet drawn the line of distinction between spirit and matter as sharply as Plato and Aristotle. The Ionian philosophers (600-500 B.C) had an animistic notion of reality, but when they asked "what is?" or "what is everything made of?", they left the mythical conception of the world, and took the position of an observing and investigating subject.

In the philosophy of Heraclit from Efesos (about 500 B.C.) it is $the$ $process$ that is the substance of reality, or in other words, change. The eternal transformation is conceived as an organizing reason, an all-embracing logos.

Also Heraclit's contemporary Pythagoras focused on the problem of change: what changes, and what is permanent during a change. The Pythagorean answer is that that which is permanent is not a substance but mathematical relations, i.e. numbers; an approach thta comes close to that of modern physics.

After the discovery that the pitch of a tone is exactly determined by the length of the vibrating string, the Pythagoreans developed a number mysticism based on the idea that the numbers are "the principles" of physical objects: to each fundamental concept corresponds a number which is its real essence. According to the Pythagoreans the number series from one to ten constitutes the elements of the world, and they defined ten fundamental pairs of opposites (limited-unlimited, odd-even, left-right, etc.). They moreover believed that the Earth is spherical and that the five known planets and the sky of fixed stars move around the middle of the universe.

In the years 500-400 B.C. the philosopher Anaxagoras in Athens preached that "everything is in everything". Another Athenian philosopher of that time was Democritus, who was the first to formulate the distinction between the phenomenon and "the thing in itself". We sense phenomena such as sounds and colours, but all that really exists, are atoms and empty space. Democritus claimed that his teacher Leukippus was the author of the causality thesis "nothing happens without a cause, everything has a cause and is necessary". If we perceive the causality thesis as a truism, it is because we are the inheritors of this philosophy, which describes the world as consisting of well ordered processes where distinct causes give rise to distinct effects.

Plato took over the Pythagorean idea of absolute truths that are unchangeable, logical and mathematical. These truths concern the higher reality, the realm of ideas, contrary to the material world which is nothing but a shivering shadow on the wall of the celebrated Platonic cave.

Platonism was taken up by the early Christianity. It was Plotinus who in the third century A.D. developed the Neoplatonic philosophy, a merging of Pythagorean and Platonic elements with Oriental mysticism. This part of Plato's thinking thus became highly influential on the medieval thinking, but as his other writings became known during the Renaissance, also the logical-mathematical side of Platonism eventually became influential.

The Aristotelian doctrine had an even greater influence on the medieval mind than Platonism. Aristotle's universe was less coherent than Plato's, inhabited by myriads of different potentialities. While the Christians answered the question why things are as they are by "it is the will of God", Aristotle's answer was that "it is their nature". Stones fall to the ground because it is their nature to do so, they have a potentiality to fall, just like trees grow upwards and night follows day. Contrary to Plato's two different worlds, the world of ideas and the world of sensations, Aristotle's universe consists of a series of disparate objects that all strive to make their ideas real. Everything that is in between the initial $materia$ $prima$ and the pure forms, is relative form and relative matter. All phenomena can be explained by the teleology of this world: everything moves towards its highest destination, the cause so to speak pushes forward the course of events. The pure form is the highest goal, and also what sets everything into motion, by being the point towards which everything aims. Hence it cannot itself be in motion - this ``principle of immobility'' was later identified with God.

According to Aristotle the motion of celestial bodies is not to be understood as change, but rather as a divine, mathematical state. Mathematical precision merely belongs to the celestial sphere and is not relevant for the inexact and arbitrary earthly world.
This philosophy held Western thought in its asphyxiating grasp from the 12th century when it succeeded the Christian Neoplatonism, to the middle of the 16th century.