A turning point in history was the ancient Greek invention of scientific theory, or “natural philosophy.” This process began on the then-fertile western coast of Anatolia or Asia Minor (present-day Turkey), in the region known as Ionia. It is traditionally said to have started with Thales of Miletus, who flourished in the decades after 600 BC. Authors James E. McClellan and Harold Dorn elaborate in Science and Technology in World History, second edition:

“We do know that he came from Miletus, a vibrant trading city on the Ionian coast of Asia Minor, and that he was later crowned as one of the seven ‘wise men’ of archaic Greece, along with his contemporary, the lawgiver Solon….Thales’s claims about nature were just that, his claims, made on his own authority as an individual (with or without other support). Put another way, in the tradition stemming from Greek science, ideas are the intellectual property of individuals (or, less often, close-knit groups) who take responsibility and are assigned credit (sometimes by naming laws after them) for their contributions. This circumstance is in sharp contrast with the anonymity of scientists in the ancient bureaucratic kingdoms and, in fact, in all pre-Greek civilizations.”

Anaximander of Miletus was a Greek philosopher in the sixth century BC and a pupil of Thales. He wrote treatises on geography and cosmology and believed eclipses to be the result of blockage of the apertures in rings of celestial fire. Anaximenes of Miletus was another prominent Pre-Socratic philosopher and a younger contemporary of Anaximander. Together they contributed to the transition from magical explanations of nature to non-magical ones in ancient Greece. Anaximenes thought that the Earth was flat, a view that was challenged before 500 BC by the mathematician Pythagoras and his followers, the Pythagoreans.

The Milesian thinkers used logic and reason to criticize the ideas of other individuals and saw the need to defend their theories, thus beginning a tradition of rational and critical assessment which remains alive to this day. It appears as if these pioneering Ionian philosophers identified the basic structure of the universe as material. Thales seems to have suggested that there must be something underlying matter in the universe, out of which everything else is composed. His ideas were developed further by his successors. Thales suggested that water was the primary substance whereas Anaximenes believed air to be the primeval element.

The philosopher Heraclitus flourished in the years before and after 500 BC. According to him the heavenly bodies are bowls filled with fire; an eclipse occurs when the open side of a bowl turns away from us. He argued for a world without beginning or end, of constant change as well as stability. According to Plato, Heraclitus was the first person to compare our world to a river and the inventor of the famous maxim that we can never step into the same river twice.

Heraclitus held that change is perpetual, that everything flows. Parmenides, a Greek philosopher from Elea in southern Italy, in the decades after 500 BC countered with the radical notion that change is an illusion. Parmenides held that the multiplicity of existing things, their changing forms and motion, are simply different appearances of a single eternal reality. He adopted the radical position that change is impossible. His doctrine was highly influential; others felt compelled to argue against it. The Heraclitean-Parmenidean debate raised fundamental questions about the senses and how we can know things with certainty.

Zeno of Elea (ca. 490-425 BC) was a Greek mathematician and philosopher and a pupil of Parmenides. Zeno’s paradoxes were important in the development of the notion of infinitesimals. Anaxagoras and the Pythagoreans, with their development of incommensurables, may have been the targets of his arguments. If you believe Plato, Zeno and Parmenides visited Athens around 450 BC where they met the young Socrates. Whether this meeting actually took place is not universally accepted by historians, but it could have happened. In ancient India, the Jains and others did philosophical work on the concept of infinity. In Europe, Zeno’s work on the subject had repercussions right down to the invention of set theory by the German mathematician Georg Cantor (1845-1918) in the late nineteenth century AD. As historian David C. Lindberg states in The Beginnings of Western Science:

“These theories of Anaximander and Heraclitus do not seem particularly sophisticated (fifty years after Heraclitus the philosophers Empedocles and Anaxagoras understood that eclipses were simply a case of cosmic shadows), but what is of critical importance is that they exclude the gods. The explanations are entirely naturalistic; eclipses do not reflect personal whim or the arbitrary fancies of the gods, but simply the nature of fiery rings or of celestial bowls and their fiery contents. The world of the philosophers, in short, was an orderly, predictable world in which things behave according to their natures. The Greek term used to denote this ordered world was kosmos, from which we draw our word ‘cosmology.’ The capricious world of divine intervention was being pushed aside, making room for order and regularity; chaos was yielding to kosmos. A clear distinction between the natural and the supernatural was emerging; and there was wide agreement that causes (if they are to be dealt with philosophically) must be sought only in the natures of things. The philosophers who introduced these new ways of thinking were called by Aristotle physikoi or physiologoi, from their concern with physis or nature.”

The Pythagoreans pioneered the mathematical approach to nature. Their approach was in stark contrast to that of the materialists, among whom the atomists were most prominent. The materialism of the sixth century BC was extended in the fifth century by the atomist Leucippus and his pupil Democritus (ca. 460 BC-ca. 370 BC), who argued that all matter is made up of imperishable, indivisible elements of different sizes and shapes called atoma or “indivisible units.” During collisions they rebound or stick together because of hooks and barbs on their surfaces. Underlying the changes in the perceptible world there was thus both constancy and change; change was caused by different combinations of permanent atoms.

Atomism was supported by the Greek philosopher Epicurus (341-270 BC). His ethical system proved popular and was influential over the next centuries and into the Roman era. Epicureanism advocated a materialistic philosophy where good was identified with friendship, pleasure and the absence of pain and banished fear of the gods, death and eternal punishment.

These speculations about the physical nature of matter culminated in the influential ideas of the philosopher Empedocles (ca. 490-430 BC), who believed that all substances are composed of four elements: air, earth, fire and water. According to legend he was a self-styled god who flung himself into the crater of Mount Etna to convince his followers that he was divine.

In the fifth century BC, the physician Hippocrates and his followers correlated the four elements of Empedocles with four bodily humors: blood, phlegm, yellow bile and black bile. This humoral doctrine was supported by the Greek physician Galen in the Roman Empire and remained highly influential well into early modern Europe. Traditional medicine everywhere stressed general health maintenance through regulation of diet, exercise and lifestyle.

According to scholar Roy Porter in his book The Greatest Benefit to Mankind: A Medical History of Humanity, “From Hippocrates in the fifth century BC through to Galen in the second century AD, ‘humoral medicine’ stressed the analogies between the four elements of external nature (fire, water, air and earth) and the four humours or bodily fluids (blood, phlegm, choler or yellow bile and black bile), whose balance determined health. The humours found expression in the temperaments and complexions that marked an individual. The task of hygiene was to maintain a balanced constitution, and the role of medicine was to restore the balance when disturbed. Parallels to these views appear in the classical Chinese and Indian medical traditions.”

The atomists had responded to the challenge from the Milesian philosophers by stating that the material world is composed of tiny particles, but they faced the challenge of explaining how these random atoms could assume any lasting, coherent pattern or structure in nature. Their theories were criticized by Aristotle for some logical inconsistencies and for their seeming inability to explain qualities such as color, taste, odor etc. The belief in atomism was not shared by most Aristotelians and remained a minority view among the ancient Greeks.

Atomism experienced a renaissance of sorts in seventeenth century Europe. The breakthrough for “modern” atomism, now with more experimental evidence in its favor, took place in the nineteenth century AD, staring with the English chemist and meteorologist John Dalton (1766-1844) in the early 1800s. The first subatomic particle, which proved that atoms were not truly “indivisible” after all, was the electron, finally identified in 1897 by the Englishman Joseph John “J. J.” Thomson (1856-1940). Thomson’s student, the New Zealand-born physicist Ernest Rutherford (1871-1937), discovered the atomic nucleus and the proton a few years later, and the English physicist James Chadwick (1891-1974) discovered the neutron in 1932. Later in the twentieth century, many other subatomic particles were identified.