Though we cannot know these objects as things in themselves, we must yet be in a position to at least think [of ] them as things in themselves; otherwise we would be landed in the absurd conclusion that there can be appearance without anything that appears.
Immanuel Kant1
I want to reassure you that this business about perception and the nature of external reality which so exercised Democritus is not some philosophical mire of our own making. It is not something from which we can extricate ourselves only by engaging in seemingly interminable nit picking. It is a fundamental problem that will have a profound impact on our understanding of matter and it will be a recurring theme. I fear that anyone suffering the delusion that science is free from this kind of philosophical wrangling is most likely in for bit of a shock.
I propose in this chapter to move on quite quickly from the ancient Greek atomists to some of the great philosophers of the seventeenth and eighteenth centuries. Now, in doing so I don’t want to give you the impression that nothing of consequence was discussed, debated, or written by philosophers in the sixteen centuries that passed in between. But I think it’s fair to say that much of the attention of Western philosophers during this period was absorbed by the challenge of reconciling the philosophies of ancient Greece and Rome with the theologies of the ‘Abrahamic’ religions of Christianity, Judaism, and Islam.2
Some of the tenets of ancient Greek philosophy were preserved, even as the Roman Empire began its slow decline, initially by a few scholars with some facility in the Greek language. Not all of these scholars were wholly sympathetic, however. For example, the second-century ce Christian philosopher Quintus Septimius Florens Tertullianus (Tertullian, sometimes referred to as the ‘father of Western theology’), despised Greek philosophy, declaring the Greeks to be the ‘patriarchs of heretics’.3
The Greeks’ pronouncements on the nature of the soul, resurrection, and the creation simply didn’t fit the demands of a theology based on the notion of an all-powerful, omniscient, omnipresent, Christian God. It was perhaps inevitable that philosophical inquiry into the nature of the material world would get dragged into the debate. ‘Natural philosophy’ became inextricably tangled with theological questions, such that the lines we draw today between these disciplines became greatly blurred, or even non-existent.
But the emphasis on scholarship slowly returned, first through schools established by monasteries and cathedrals, some few of which eventually developed in the twelfth and thirteenth centuries into universities. The rise of academic philosophy and theology helped to renew interest in the ancient Greeks, even though formal teaching of some of their works was generally forbidden (e.g., the teaching of Aristotle’s metaphysics and natural science was prohibited by the statutes of the University of Paris in 1215).
This burgeoning interest in a selection of the ancient Greek texts proliferated new translations. By the middle of the thirteenth century the climate had changed sufficiently to allow the Italian Catholic priest and theologian Thomas Aquinas to set about the task of rehabilitating Aristotle. The resulting ‘Thomist’ philosophy is Aristotle blended with many other sources, ancient and medieval. This was only a partial or selective rehabilitation: Aquinas served two spells as regent master of theology at the University of Paris, so Thomism is really a theology or a philosophy with a distinctly Christian gloss. In this way, Aristotle’s pronouncements on the nature of matter, his Earth-centred cosmology based on perfect circular motion, caused by a prime mover, became enshrined in Christian religious orthodoxy.
So, what did Aristotle have to say about the nature of material substance? He had struggled to reconcile Democritus’ rather passive, unchanging atoms with the observation of a very lively, actively changing material reality. And, as we have seen, he rejected completely the notion of the void. The atomists’ rebuttal of Parmenides and Zeno arguably required that space and time should also be conceived to be ‘atomic’ in nature, with ultimate limits on how finely units of space and units of time can be divided.* Aristotle preferred to think of space and time as continuous, and any object taking up room in a continuous three-dimensional space is in principle infinitely divisible, thereby making atoms impossible. But he was nevertheless still sympathetic to the idea that a substance could be reduced to some kind of smallest constituent. It was just that the atomists had gone too far. They had been too reductionist in their approach.
His solution was an alternative structure based on the idea of natural minima, meaning the smallest parts into which a substance can be divided without losing its essential character. Now, natural minima are in principle not atoms, at least in the sense that the Greek atomists understood the term. Natural minima can in principle be divided. It’s just that when they are divided beyond a certain limit they no longer represent or are characteristic of the original substance. The solid wood of the holed fishing boat is divided into particles of sawdust, but the sawdust is still wood. Keep dividing the sawdust further and we eventually cross a threshold to something that is no longer wood.
Aristotle also endowed naturally occurring objects with form, a theory adapted from the teachings of his mentor, Plato. A tree consists of natural minima and possesses the form of ‘tree-ness’. It is its form that makes an object what it is and governs its properties and behaviour. In the interpretation fashioned by Aquinas, this became a substantial form, which is not reducible to component parts. A tree is a tree, and cannot be reduced to component parts, such as trunk, branches, leaves, and so on, as the components are not ‘trees’. Cut the tree down and use the wood to build a fishing boat and the substantial form of ‘tree-ness’ is lost.
This construction was a gift to medieval theologians. Humans can build boats, but only God can make a tree. The fusion of matter and form to make objects is readily applied also to the fusion of body and soul. It was a handy explanation for transubstantiation, the bread and wine used in the sacrament of the Eucharist carrying the substantial forms of the body and blood of Jesus Christ.
It would be all-too-easy to view this as a period of intellectual activity that was in some way stifled or imprisoned by some Machiavellian creatures of the established Church. It’s certainly true that whilst philosophers were generally free to think what they liked, they were not free to write and disseminate what they thought without fear of accusations of heresy and all that this entailed. But the seeds of slow and quiet intellectual reformation were actually sown by leading Church figures, including several fifteenth- and sixteenth-century Popes: Pius II, Sixtus IV, and Leo X.
These were Renaissance humanists, who helped to establish studia humanitatis, what we know today as ‘the humanities’, a rediscovery of the learning and values of ancient Greece and Rome. The study of the humanities became possible in ways that were very different from the narrow prescriptions of medieval scholarship. The influence of Renaissance humanism can’t be over-stated. It was to become one of the most important forces for change in human intellectual history.
In January 1417, the Italian scholar and manuscript-hunter Gian Francesco Poggio Bracciolini discovered a copy of Lucretius’ De Rerum Natura languishing in a German monastery. He sent a copy to his friend Niccolò de’ Niccoli, known to us today for inventing cursive, or italic, script. De’ Niccoli agreed to transcribe it, though he appears to have sat on it for at least twelve years. Eventually, more copies were made (more than fifty copies from the fifteenth century have survived to the present day), and Johannes Gutenberg’s ingenious invention helped spread Lucretius’ poem across Europe.
But it was not what Lucretius had to say about the atomic theory of Epicurus that fired imaginations in this period. More interesting to fifteenth-century readers was what he had to say about the ‘natural order’; the basis for philosophizing without the constant intervention of one (or more) deities, a natural order which appears not to have been designed specifically with humans in mind. Lucretius also talked about the death of the soul and the fallacy of the afterlife, and of the cruelty and superstitious delusions of organized religion.*
Safe to say, this did not play well. Printed editions of Lucretius’ poem appeared with warnings and disclaimers, and in the early sixteenth century it was banned from Italian schools. The Dominican friar Giordano Bruno was a vocal advocate of the Epicurean philosophy and of a cosmology that placed the Sun—not the Earth—at the centre of the universe, as proposed by Nicolas Copernicus in his book De Revolutionibus Orbium Coelestium (‘On the Revolutions of the Heavenly Spheres’), published in 1543. On his ill-advised return to Italy from wandering around Europe, Bruno was arrested in May 1592 and imprisoned by the Inquisition. He refused to recant and was burned to death in February 1600.
It was clear that philosophers still needed to proceed with great caution. But by the seventeenth century they had sufficient intellectual freedom to establish structures for interpreting and understanding the physical world that were once again largely materialistic or mechanistic in nature. This was a world that might have been designed by God but whose mechanisms appeared free of overtly divine intervention. And they also gained the freedom to begin the slow process of disentangling philosophy from theology.
Again, it is probably a mistake to think of this as a triumph of science or rational thinking over religious superstition. Many of the seventeenth-century philosophers who ushered in this new ‘age of reason’ were motivated by efforts to understand the world that God had designed and created and to reconcile their conclusions with Christian doctrine. These were still philosophers, forming two broad and overlapping groups that we tend to think of today as ‘mechanical’ and ‘classical modern’.
Among the mechanical philosophers, we would count Francis Bacon (born 1561), Galileo Galilei (1564), Johannes Kepler (1571), Pierre Gassendi (1592), Robert Boyle (1627), Christian Huygens (1629), and Isaac Newton (1642). Among the classical modern philosophers, we would count René Descartes (1596), John Locke (1632), Baruch Spinoza (1632), Gottfried Leibniz (1646), George Berkeley (1685), and, stretching into the eighteenth century, David Hume (1711) and Immanuel Kant (1724). Today we tend to think of the former group as ‘scientists’ (or at least heralds of the scientific revolution) and the latter as ‘philosophers’. In truth they form a near-continuous spectrum, varying only in the nature and methods of their inquiry. Many of the ‘scientists’ engaged in philosophical (and theological) reflection and many of the ‘philosophers’ engaged in experiment, or at least acknowledged the conclusions of experimental science. Descartes was also a mechanical philosopher.*
Boyle was a noted experimentalist, conducting studies in medicine, mechanics, hydrodynamics, and the properties of gases.† He also dabbled in alchemy and was an ardent Christian, advocating the ‘argument from design’ for the existence of God, encouraging the translation of the Bible and the dissemination of its messages.4 He was an admirer of James Ussher, the Archbishop of Armagh, whose literal analysis of the Book of Genesis had led him to conclude that the Earth had been created by God in 4004 bce, on 22 October, around 6 pm.
Of the early mechanical philosophers, Gassendi and Boyle were perhaps the most influential in reintroducing the idea of atoms (Descartes couldn’t get past his dislike of the void). Gassendi embarked on an ambitious attempt to reconcile the Epicurean philosophy with Christianity, and his atoms were recognizably those of Epicurus. But Boyle’s atoms were rather different.
In The Scientist’s Atom and the Philosopher’s Stone, first published in 2009, contemporary philosopher of science Alan Chalmers carefully traces the evolution of the sanctioned, orthodox Aristotelian description of matter into the more recognizable atomism adopted by the mechanical philosophers at the beginning of the scientific revolution. A key figure in this evolution was Paul of Taranto, a thirteenth-century Franciscan monk, who is believed to have written treatises on alchemy under the pen-name of Geber, ‘borrowing’ the name from a famous tenth-century Muslim alchemist Abu Mūsā Jābir ibn Hayyān (in the West ‘Jābir’ was Latinized as ‘Geber’). The writings of ‘pseudo-Geber’, together with the medieval adaptation of Aristotle’s natural minima and the atomic theory of Democritus, were powerful influences on Daniel Sennert, a renowned seventeenth-century German physician and professor of medicine at the University of Wittenburg.
Sennert also dabbled in alchemy. In one famous experiment he reacted metallic silver with aqua fortis (nitric acid) to produce a new compound which is not simply a mixture of the initial ingredients (we would today identify the product of this chemical reaction as silver nitrate). The resulting solution could be filtered without leaving the residue that would be expected if particles of silver had simply become dissolved in the acid. Adding salt of tartar (potassium carbonate) to this solution caused another compound (silver carbonate) to precipitate. Filtering, washing, and heating this precipitate would then recover the metallic silver, in its ‘pristine state’.
This kind of experiment causes all sorts of problems for Aristotle’s theory of forms. It is clear that the natural minima of silver persist unchanged through all these chemical transformations, since they are present at the start and can be recovered at the end. Sennert concluded that the natural minima of silver serve as components in the natural minima of each of the compounds formed through the sequences of chemical transformations. But these compounds are substances in their own right, with properties that are quite distinct from those of the starting materials. He was therefore obliged to admit the possibility of a hierarchy of forms. In other words, the forms themselves can be transformed and new forms created.
Although he did not openly acknowledge it in his writings, Boyle’s views owed much to Sennert. He dismissed as unnecessary all reference to the Aristotelian forms, and argued that the properties and behaviour of a material substance can be traced to the natural minima from which it is composed. Boyle rarely used the word ‘atom’, although this is undoubtedly where he was heading (and to simplify the following discussion I’ll revert to ‘atoms’ in preference to natural minima).
Boyle accepted that the atoms are too small to be perceived but assumed them to possess size (and hence weight), shape, and motion. They are physically indivisible or at least, like Sennert’s silver atoms, they remain intact through chemical and physical transformations. This looks very much like the atomism of Democritus and Epicurus, although Boyle had arrived at this structure in his own way and on his own terms. Boyle’s atomism was not a ‘rediscovery’ of the ancient theory.
The mechanical philosophers drew on the talents of a new generation of artisan instrument-makers, developing the technology needed for systematic observation or experimentation, such as telescopes, microscopes, clocks, and vacuum pumps. Boyle’s experiments with an air pump (built by his laboratory assistant at Oxford, Robert Hooke) led him to deduce the relationships between gas pressure and volume that are the basis for Boyle’s law. Squeeze a gas into a smaller volume whilst maintaining a constant temperature and the pressure of the gas increases in direct proportion. Allow a gas to expand into a larger volume whilst maintaining a constant temperature and the pressure of the gas falls in direct proportion. If the temperature is fixed, pressure multiplied by volume is constant.
But these kinds of experiments were not so refined that they resolved the problems that had confronted the ancient Greeks. Just how do you draw conclusions about atoms that by definition cannot be observed directly in the properties and behaviour of matter on a large scale?
Boyle’s solution was to suggest that properties and behaviour observed to occur ‘universally’ in matter of all kinds can logically be ascribed to the atoms themselves. In other words, what we see in our large-scale, macroscopic world of experience applies equally to the microscopic entities from which substance is composed. This is arguably no more advanced than Lucretius, who suggested we maintain our mechanical sensibilities when observing natural phenomena. But the microscopic atoms are still required to possess properties (such as indivisibility) that are simply absent from our macroscopic world, and so these are properties for which the new experimentalists could establish no evidence.
As the mechanical philosophers fussed with their instruments, the classical modern philosophers set about the task of defining what, if anything, could be learned about the material world from reason alone. Descartes set out to build a new philosophical tradition in which there could be no doubting the absolute truth of its conclusions. From absolute truth, he argued, we obtain certain knowledge. However, to get at absolute truth, Descartes: ‘… thought I ought to … reject as being absolutely false everything in which I could suppose the slightest reason for doubt’.5 This meant rejecting all the information about the world that he received using his senses.
He felt he could be certain of at least one thing: that he is a being with a mind that has thoughts. He argued that it would seem contradictory to hold the view that, as a thinking being, he does not exist. Therefore, his existence was also something about which he could be certain. Cogito ergo sum, he famously concluded: ‘I think therefore I am.’
Having proved his own existence (at least to himself), Descartes went on to present a number of proofs for the existence of God, conceived as an all-perfect being which, he established by logical reasoning, must be a force for good. Now, if Descartes’ ideas of objects existing in the world are deceptions, then this would mean that God was deceiving him or allowing him to be deceived. If we take deception to be a source of imperfection, then this would seem to contradict the idea of God as an all-perfect being. Descartes concluded that his ideas of physical objects must therefore be the direct result of the existence of these objects in the external world.
But, just as the ancient Greeks had done, he recognized the distinction between the objects themselves and the sensory perceptions they create, and that the latter do not necessarily convey an accurate representation of the objects ‘in reality’. We perceive different colours, smells, tastes, sounds, and sensations such as heat, hardness, and roughness. The sources of all these different perceptions would seem to be created by the objects themselves, but this does not mean that they are intrinsic properties of the objects.6
This distinction was greatly sharpened by the English philosopher John Locke in 1689. He argued that, no matter how finely we might divide a substance, what we get retains certain intrinsic, or primary, qualities, such as shape, solidity, extension (in space), and motion. If the ultimate constituents of matter are atoms, then we can expect that these atoms will possess the primary qualities.
But there are also secondary qualities that are not intrinsic but which result from interactions with our sensory apparatus: eyes, ears, nose, tongue, and skin. Individual atoms might possess shape and solidity and so on, but they do not possess colour, sound, or taste. Colour, for example, is the result of an (unspecified) interaction between the atoms and our eyes.7
This seems to make sense, but philosophers like nothing better than a good argument, and they were far from done. Try as he might, the Irish philosopher George Berkeley couldn’t see the difference between primary and secondary qualities. Whilst it might be possible to distinguish between them, as far as he was concerned it is practically impossible to separate them. If we can’t conceive of an object possessing shape and solidity without at the same time possessing colour or sound, then this suggests that, despite the distinction, primary and secondary qualities have much the same status. Berkeley was happy to concede that secondary qualities exist only in our minds. What he was saying is that primary qualities exist only in our minds, too.8
The Scottish philosopher David Hume was inclined to agree. He concluded simply that we have no means of knowing what, if anything, exists beyond our capacity to experience it through perception and so the question is, broadly speaking, meaningless. His strategy was to consign all speculation about the nature of reality to metaphysics (meaning literally ‘beyond physics’) and to adopt a fairly negative attitude to any claim to knowledge achieved through metaphysical reasoning and speculation. In arguing this position Hume helped to establish a philosophical tradition known as empiricism.
In an empiricist philosophy, knowledge of the world that is not gained through direct experience is rejected as meaningless metaphysics. This does not necessarily mean that there is no such thing as a reality existing independently of perception: the Moon is still there, even if nobody looks. But it does mean that we might have to manage our expectations. At best, we gain knowledge of an empirical reality—the reality manifested as effects that we can directly perceive, or make measurements on.
The great German philosopher Immanuel Kant was deeply influenced by Hume’s work, but he came to deny Hume’s conclusion that it is therefore impossible to acquire knowledge through anything other than experience. He differentiated between what he called noumena, the objects or things-in-themselves, and phenomena, the things-as-they-appear as we perceive and experience them.
Hume would likely have argued that noumena are metaphysical and meaningless, but Kant argued that, through phenomena, the noumena impress themselves upon our minds because our minds have what he called sensible intuitions, specifically of space and time. We construct space and time in our minds in order to make sense of the things we perceive in the external world around us.
Kant claimed that it made no sense to deny the existence of things-in-themselves, as there must be some things that cause appearances in the form of sensory perceptions (there can be no appearances without anything that appears). But he agreed with Hume that whilst the things-in-themselves must exist, we can in principle gain no knowledge of them.
What does this kind of logic mean for atoms? It can be argued that these entities produce appearances both obvious (as Lucretius had noted) and more subtle (as the alchemists and early mechanical philosophers had deduced). But it is a simple and rather stubborn fact that they cannot be perceived. Some later scientists and philosophers who would declare themselves to be staunch empiricists, such as Austrian physicist Ernst Mach, would come to dismiss atoms as metaphysics and deny that they really exist. It was certainly true that, for as long as experimental science was unable to access phenomena that could be attributed more directly to atoms, they would remain, at best, a ‘working hypothesis’.
This was nevertheless good enough for many of the practically minded mechanical philosophers, who had long before embarked on a voyage of empirical discovery. They tended to their instruments and conducted careful, systematic experiments, varying first this, then that. And there was something else. They discovered that the laws of nature are written in the language of mathematics.