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rather than purpose, is strikingly similar to that of nineteenth-century physics. This similarity is no accident, of course, since the modern concept is partly derived from Empedocles.

      Empedocles’ view of the world does, however, diverge radically from the modern in many ways: he also saw the two forces, love and hate, in a religious sense, as a struggle between good and evil (with the four elements each identified with a different god). His scheme of things also differs from ours in that his elements correspond more to the modern phases of matter (solid, liquid, gas, and plasma) rather than to modern elements (such as hydrogen, oxygen, nitrogen, and carbon). This difference partly arises from the fact that Empedocles appears to have rejected the idea of empty space – the void or vacuum – a space where there was nothing, no elements or anything else. Since he had shown that air was a substance, he saw no reason to believe in empty space between the elements. Thus he conceived of the elements as homogenous substances, which blended together when mixed, like different-coloured paints.

      Earlier thinkers (such as Anaximenes) and later thinkers (such as Democritus) took the more modern view that a substance consists of a vast number of small particles separated by empty space, and conversion from liquid to gas is not due to a change of elements, but rather to the elements moving much further apart. Thus, ice consists of water molecules held rigidly together, while liquid water consists of the same water molecules flowing over each other, and steam, or completely evaporated water, consists of the same water molecules very far apart. The Atomists – Leucippus and Democritus (c. 460–370 BC) – pushed this view of the world to its most materialistic extreme, by taking Empedocles’ world, ridding it of its religious components, but adding the void. Thus, their view was that there was nothing in the world except a vast number of tiny particles (atoms) moving through empty space. Each of the four elements had a different shaped particle, and this shape determined the properties of the element. This explanation of the world had great advantages over the no-void view, because it could explain easily how the elements could mix and then separate: particles simply passed between each other; whereas this was hard to explain if there was no empty space between elements. Similarly, Empedocles had considerable difficulty explaining why the millions of things in the world had such startlingly different properties, if only differing in the proportions of the four elements. Why should a difference in proportions cause new properties? Democritus (and modern science) could explain this by the arrangement of the atoms within the object. New properties arose from new spatial arrangements or configurations of the atoms. There were an infinite number of ways of arranging atoms of four elements, and consequently an infinite number of possible things or materials. This is the essential secret of the success of modern chemistry and biology: explaining the properties of things in terms of the microstructure of the elements of which they consist. Unfortunately for the Atomists, the technological means did not exist in Greece to probe the microstructure of things, and thus test their theories.

      We have been pursuing these ideas about matter, because they lie at the root of modern notions of energy. But Empedocles was far more than a creative physicist (physis was Greek for nature), he was also an inventive biologist (bios, Greek for life). According to Empedocles, the body’s flesh and blood consisted of equal proportions of all four elements, and these attracted similar elements from the environment. Thus, the same four elements constituted non-living and living matter, mind and the immortal gods. The blood circulated from the heart to the surface of the body, where air was taken in through the pores, and back again, alternately expelling and drawing in air. The motion of blood in and around the heart created thought, and so the heart was seen as the organ of consciousness. But Empedocles had a very concrete view of consciousness, seeing for example, thought as simply blood in motion. Perception occurred by elements in the blood meeting and mingling with the same elements in the environment. An external object was perceived by some elements from it entering the body and meeting the corresponding elements in the body, and their meeting or mingling was perception. Nutrition occurred through direct assimilation, that is, the elements of the body attracted similar elements in the environment to them, and these new elements fitted in place to form the growing body.

      The theory of the four elements was astonishingly popular and long-lived, lasting from the fifth century BC until the chemical revolution of the seventeenth century. Yet it is hard to see quite why thinkers stopped at only four elements. Aristotle suggested a fifth – the ether – to compose all extraterrestrial things. The Chinese used five elements also (or phases): water, earth, fire, metal and wood. In modern science we have about 100 different chemical ‘elements’, which can combine to give an infinite number of possible molecules. But at the beginning of the twentieth century the Cambridge physicists J. J. Thomson, Ernest Rutherford, and James Chadwick discovered that these chemical elements were not in fact elements in the classical sense (fundamental and indestructible particles of matter), because they were destructible and composed of three simpler, indestructible particles – the proton, electron, and neutron. And these three particles were later found to interact via a fourth (short-lived) particle – the photon. Therefore, Empedocles’ four elements and two forces theory is, in outline, not that dissimilar to much more modern theories of the Universe.

      Hippocrates (c. 460–377 BC) is called the founding father of medicine, and his theories of disease, cure and physiology influenced medicine and biology up until the eighteenth century. However, his own life is so mythologized that it is impossible to distinguish the basic events of his life, or even whether he really ever existed. According to legend, Hippocrates was a physician from Cos, and he practised medicine in Thrace, Thessaly and Macedonia, before returning to Cos to found a school of medicine. This school flourished from the late fifth century to the early fourth century BC, producing a vast number of highly original medical texts. Copies of around seventy of these books survive. These were conventionally attributed to Hippocrates, although he probably wrote none of them himself. The defining characteristic of Hippocratic medicine was its rejection of religious and philosophical explanations of disease, and its search for an empirical and rational basis for treatment.

      Since prehistoric times, disease had been thought caused largely by gods, evil spirits, or black magic. A cure could thus be effected by ejecting the sin, spirit, or magic from the sufferer via various processes of purification. In Greece, traditional medicine was practised by priest-physicians in temples dedicated to the god Asclepius. In these temples of health, disease was apparently diagnosed partly on the basis of dreams and divination, and partly on the symptoms. Cures were half rituals and spells, and half based on fasting, food, drugs and exercise. According to later legend, Hippocrates was descended from the god Asclepius and brought up on Cos as son of a renowned priest-physician. The relationship between secular medicine (represented by Hippocrates) and religious medicine (based on faith healing or magic) in ancient Greece is difficult to discern, although apparently not as antagonistic as today.

      Hippocrates and his followers accepted the doctrine of the four elements as an explanation for the natural world, but their concern as doctors was with disease’s causes and treatment. The four elements – earth, fire, air, and water – cannot be seen in anything approaching a pure form in or on the body. Also they knew relatively little about the inside of the body, because dissection was prohibited on both religious and ethical grounds. So the Hippocratics concerned themselves with what they could see and use in the diagnosis of disease, particularly the bodily fluids: blood, saliva, phlegm, sweat, pus, vomit, sperm, faeces and urine. Gradually the doctrine evolved that there were only four basic fluids (humours): blood, phlegm, yellow bile, and black bile. Blood can appear in cuts, menstrual flow, vomit, urine or stools. Phlegm is the viscous fluid in the mouth (saliva) and respiratory passages and comes out through the mouth and nose in coughs and colds. Yellow bile is the ordinary bile secreted by the liver into the gut to aid digestion; it is a yellow-brown fluid that colours faeces. The identity of black bile is not entirely clear, perhaps originally referring to dark blood clots, resulting from internal bleeding, which may appear in vomit, urine or faeces. However, the four humours did not only refer to these particular fluids, but were thought to be the body’s basic constituents. Health was thought to be due to the balance of these humours, and ill health an imbalance of the humours. Epilepsy was, for example, thought to be caused by an excess of phlegm in the brain blocking the flow of pneuma (vital spirits) to the brain. Thus treatment sought to restore the balance between the humours by removing the humour that was

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