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inquiry, the Middle Ages did produce a collection of notable and original thinkers who contributed to a gradual reawakening of rationality. Together, these men led the way to the Renaissance and the full flowering of innovative science that followed.

      Still wrapped up in the need to marry natural philosophy with theology, the thinkers of this period – who became known as the Scholastics, the most famous of whom were St Thomas Aquinas and Albertus Magnus – stuck to the traditional Aristotelian line, shunning experiment. However, they did champion the search for truth outside the limited realm of pure theology. Although they maintained a firm belief that man was the central object of Creation and that the universe was designed for man by God, they had progressed to the idea that the study of Nature and the physical world could lead to greater theological enlightenment. It was not until the deaths of Aquinas and Albertus Magnus (towards the end of the thirteenth century, some seventy-five years after Aristotle had been reintroduced into Europe) that the work of the great Oxford scholar Roger Bacon began to erode the restrictions of Scholasticism.

      In some ways Bacon was a man born ahead of his time. Although he subscribed to many traditional beliefs of the Scholastics, he was the first to see the usefulness of experiment and he composed three far-sighted tracts – Opus Majus, Opus Minor and Opus Tertium – which outline his philosophy and his experimental techniques in a range of disciplines. This effort established Bacon’s reputation for posterity, but did little for him during his lifetime. Viewing his work as anti-Establishment and its anti-Aristotelian elements as subversive, Jerome of Ascoli, General of the Franciscans (later Pope Nicholas IV), imprisoned him for life as a heretic.

      The scientific renaissance that followed Bacon’s time marks a change in philosophical beliefs every bit as significant as that in the arts. Men such as Leonardo da Vinci, who approached science from a practical standpoint, foreshadowed many of the ideas of Galileo, Kepler and Newton, but did not write up their discoveries in any coherent form. The best we have is Leonardo’s collection of notebooks, which indicate his studies and philosophies. In one sense, Leonardo was all experiment and represented the opposite extreme to the Greeks.

      Leonardo held an opposing view of motion to Aristotle. Aristotle claimed that nothing moved unless it was made to do so by God, the Unmoved Mover. Leonardo suggests the exact opposite, writing in his notebook, ‘Nothing perceptible by the senses is able to move itself … every body has a weight in the direction of the movement.’⁴ In other words, matter has an innate tendency to move in a certain direction unless stopped. This anticipates the notion of inertia first postulated by Galileo some half-century later and eventually formalised by Newton.

      Galileo, who was born in 1564 (about forty years after Leonardo’s death), is regarded by historians of science as the greatest thinker in the realm of motion and matter up to Newton’s time. It is generally agreed that his practical demonstrations paved the way for Newton’s own blend of experimental verification and mathematical integrity.

      Galileo’s work in this area was revolutionary because he was the first to devise repeatable experiments that showed that Aristotle’s ideas were quite wrong. He is probably most famous for his use of the telescope, which destroyed the traditional ideas of how the solar system is constructed (see Chapter 4), but equally important for the progress of science was his work in what became known as the science of dynamics.

      Aristotle held that bodies were either intrinsically light or heavy and they fell at different velocities because of their innate tendency to seek their natural places. In 1590 the Flemish philosopher Simon Stevin had shown that light and heavy objects falling through a vacuum reach the ground simultaneously. Galileo repeated this experiment the following year (although probably not from the Leaning Tower of Pisa as tradition had it) using a cannonball and a musket-ball and showed that the two fall at equal speed if the resistance of air is ignored.

      More importantly, Galileo suspected from this experiment that a falling body moves with a speed proportional to the time it has been falling. But, because the balls fall too quickly for the eye to measure their actual speed, he could not formulate a mathematical relationship between the speed of descent and the time it took. In order to find this relationship, he needed to conduct an experiment in which the speed of descent could be measured.

      He quickly established that, ignoring friction, an object rolling down an inclined plane acquires the same speed as it would if it was falling vertically through the same distance. This enabled him to construct a series of experiments in which he let balls roll along inclined planes and measured the time of their journey and their speeds. This confirmed that the speed of a falling object does indeed increase with the time of the fall.

      In a variation on this experiment, he allowed a ball to roll down an inclined plane and roll up another. In a further test, he allowed the ball to travel on beyond the slope along a horizontal path, where it continued steadily until slowed and eventually stopped by friction.

      It was these experiments that convinced Galileo that Aristotle’s idea of the Unmoved Mover was false. Objects do not move because they are constantly being pushed or pulled: rather, they possess inertia – an innate tendency to move unless stopped.

      This was a revolutionary notion, but his views on other questions concerning matter and energy also entitle Galileo to be seen as the first of the modernists. He rejected Aristotle’s idea of the four elements and subscribed to Democritus’s atomic theory at least three decades before it began to make a reappearance in the schemes of Europe’s leading thinkers, though he was unable to prove it. He also flew in the face of Aristotle’s insistence that objects possess integrally all the properties we sense when we observe them, declaring:

      I feel myself impelled by necessity, as soon as I conceive a piece of matter or corporal substance, of conceiving that in its nature it is bounded and figured by such and such a figure, that in relation to others it is large or small, that it is in this or that place, in this or that time, that it is in motion or remains at rest, that it touches or does not touch another body, that it is single, few or many; in short by no imagination can a body be separated from such conditions. But that it must be white or red, bitter or sweet, sounding or mute, of a pleasant or unpleasant odour, I do not perceive my mind forced to acknowledge it accompanied by such conditions; so if the senses were not the escorts perhaps the reason or the imagination by itself would never have arrived at them. Hence I think that those tastes, odours, colours etc. on the side of the object in which they exist, are nothing else but mere names, but hold their residence solely in the sensitive body; so that if the animal were moved, every such quality would be abolished and annihilated.⁵

      So, contrary to Aristotle, Galileo states categorically that there are two distinct qualities of bodies. The first may be considered primary qualities, which are inseparable from and fundamental to the nature of the object in question – what twentieth-century scientists would ascribe to the atomic structure and chemical nature of an object. The others are secondary qualities, which are interpreted by the senses of the observer.

      These revolutionary notions of Galileo’s – ideas which have perhaps been swamped by his more famous discoveries in astronomy and dynamics – greatly influenced the French philosopher René Descartes, who for a time informed Newton’s thinking on the subject of matter and the nature of the physical universe.

      Descartes is most famous today for two developments – Cartesian coordinates, which still play a key role in mathematics, and dualism, a philosophy which proposes a sharp distinction between body and soul, matter and spirit. According to Cartesian dualism, the spirit is personal and nebulous, and matter must therefore be impersonal and concrete.

      In Descartes’s image of the universe, matter is immersed in an unseen, immeasurable medium called the ether. God endowed the universe with movement at the beginning of time and allows it to run spontaneously but in accordance with his will. Because in this scheme matter fills all of space, there can be no such phenomenon as a vacuum and all motion is produced by matter impressing on other matter within the medium of the ether. Descartes expressed this in his famous theory

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