Kurt Godel (1906 – 1978) was an Austrian-American logician, philosopher, and mathematician considered to be in the same league as Aristotle as one of history’s greatest logicians. He is best known for his Incompleteness Theorem, one of the twentieth century’s most significant mathematical results. It posits that within any axiomatic mathematical system, there are propositions which can be neither proved nor disproved based on that system’s axioms. Godel was born in the Austro-Hungarian Empire, in Brno, in what is now the Czech Republic.
At age six, Godel endured a bout with rheumatic fever, an inflammatory heart disease, which left him sickly for the remainder of his childhood. The rheumatic fever bout also left him with a lifelong concern about his health that grew into hypochondria, As seen below, it eventually became a full blown paranoia that did him in. Brilliant since childhood, by 1929 he had graduated from the University of Vienna, an intellectual hub of the world in those days. He joined its faculty a year later. His brilliance, however, was marred by paranoia. It kept him at a distance from the university’s other brilliant minds, and left him convinced that the twentieth century as a whole was hostile to him.
After Godel’s Incompleteness Theorem was published, he became a celebrity within intellectual circles, and travelled to America many times in the 1930s. There, he met and befriended Albert Einstein, and began to lecture at Princeton University. When the Nazis annexed Austria in 1938, Godel’s friendship with Jewish intellectuals made him suspect. Between that and fear of conscription into the Wehrmacht, he fled to the US. There, with Einstein’s help, he got a position teaching at Princeton’s Institute for Advanced Studies, where he remained until his retirement in 1976.
Godel’s paranoia worsened as he aged, however, and he suffered bouts of mental instability. Key among them was a persecutory delusion that left him with an irrational fear of getting poisoned. As such, he would only eat food that his wife had prepared for him and then tasted first. When in 1977 she was hospitalized for six months and was unable to prepare his food, he refused to eat and literally starved to death. He was down to 65 pounds by the time he died.
“Fun” is probably not what comes to mind when most people think of Pythagoras (circa 570 – circa 495 BC), the ancient Greek philosopher whose Pythagorean theorem has tormented school children for generations untold. To be sure, he had some funny beliefs, such as a hatred of beans, because he thought that they contained the souls of the dead. Or his conviction that people lost a part of their soul whenever they farted. The thing though, is that Pythagoras did not think those beliefs were funny.
As seen further down this list, Pythagoras was dead serious about his strange beliefs. Literally, dead serious. As he fled from pursuers out to kill him, his flight path ended at a field of beans. Rather than cut through the field and come in contact with the detested beans, he turned around to face his killers, who promptly did him in. However, as seen below, Pythagoras did have a fun side, some of which was manifested in his invention of a prank cup that spilled wine on drinkers.
One fun activity that Pythagoras enjoyed was to drink wine. For that matter, so did most ancient Greeks. However, the philosopher had a pet peeve when it came to drinking: he did not like wine hogs. Specifically, he did not like it when greedy friends filled their cups to the brim, and took more than their fair share of the wine. So to teach them a lesson, he invented a special cup that came to be named after him. Superficially, the Pythagorean Cup looks like a traditional ancient Greek goblet.
Inside, however, the Pythagoras cup contains a column that sticks up the middle. One can drink from it like from any other goblet, provided one does not try to fill it to the maximum. Pythagoras designed the cup so that if a drink companion became a wine hog and tried to fill it, it would instead drain all the wine and spill it out the bottom. Presumably, with wine spilled all over him – and the hassle of wine stain removal – the greedy friend would learn a lesson about the moderation.
Pythagoras’s Cup uses the basic principle of the siphon – same as that used to drain gas out of a car’s tank with a hose. The column inside the cup has a small hole at the bottom. The hole leads to an inverted U-shaped pathway inside the column. The pathway leads up from the hole at the bottom of the cup’s interior, to the top of the column, then loops back down to another hole at the base of the cup.
When wine is poured into the cup, the column inside fills to the same level as the wine in the cup. So long as the cup’s wine level does not reach the top of the U, the Pythagorean Cup functions like any other cup. However, if the wine level tops the column, and thus the U bend within it, the cup’s special effect takes over. Soon as wine tops the U bend and spills into the part of the column headed towards the hole at the base of the cup, the cup becomes a siphon, and begins to drain. Once the siphon effect begins, it continues to drain wine until the cup is empty.
Karl Marx (1818 – 1883) was the German philosopher and radical socialist whose Communist Manifesto and Das Kapital formed the basis of Marxism, and revolutionized the world for better and for worse. Born in Prussia, he experimented with sociopolitical theories in university, and by the 1840s had become a radical journalist. His writings were viewed as dangerous by the authorities, and in the span of a few years he was expelled from Germany, France, Belgium, then Germany again, before he found refuge in London. There, he settled and lived for the remainder of his life. Karl Marx’s father was a successful lawyer, a man of the Enlightenment, and a passionate advocate for Prussian reform. He had converted from Judaism to Lutheranism to avoid legal restrictions that barred Jews from high society.
Karl received a liberal education in a school whose enlightened leanings made it suspect in the eyes of the reactionary authority. So they raided it in the 1830s, confiscated writings deemed subversive from its library, and forced changes in the academic staff. Marx’s early years of higher education were marked by poor grades, jail for drunkenness, riotous behavior, and general rowdiness, before he buckled down to serious study of the law and philosophy. He was strongly influenced by the philosopher Hegel. He joined a radical student group known as the Young Hegelians, which began his transformation into a radical, and eventually revolutionary, thinker.
Karl Marx Was Unsentimental to the End, and Thought Last Words Were for Fools
Karl Marx received a doctorate in 1841. However, he could not get a teaching job because of his politics. So he took to journalism. Within a year, however, his newspaper was suppressed, and he was forced to move to Paris and the relatively freer French environment. In Paris, he met Freidrich Engels, and the two developed a friendship and began a collaboration that revolutionized the world. In 1845, the Prussians pressured the French to expel Marx. So he moved to Belgium, where he founded a correspondence committee to link European socialists. That inspired English socialists to form the Communist League, and ask Marx and Engels to write a platform for their party. The result was the Communist Manifesto, published in 1848.
Shortly thereafter, Marx was expelled from Belgium. He went to France, which also expelled him. He returned to Prussia, but by then he had been stripped of his citizenship, and the authorities refused to re-naturalize him. Eventually, he ended up in London in 1849. He spent the remainder of his life writing, and in 1867 the radical philosopher published Das Kapital. Twinned with the earlier Communist Manifesto, it became the philosophical bedrock of Marxism and communist theory. On his deathbed in 1883, as he expired from pleurisy, he was solicited for final words. His reply, before he breathed his last, was: “Go on! Get Out! Last words are for fools who haven’t said enough!”
The Roman Philosopher Who Sparked the Renaissance Centuries After His Death
Marcus Tullius Cicero (106 – 43 BC), considered Rome’s greatest orator, was a statesman, philosopher, scholar, lawyer, and writer who served as consul in 63 BC. Throughout his career, he tried in vain to uphold republican principles as the Roman Republic tore itself apart in civil wars during its final years. He had much greater impact and success influencing Western thought for centuries. The rediscovery of his writings more than a millennium after his death is credited with sparking the Renaissance.
Cicero was born into a wealthy equestrian family in Arpinum, and was sent to study law in Rome as a youth. His brilliant defense of a Sextus Roscius in 79 BC against trumped up charges of parricide established his reputation as a lawyer and began his rise in Rome. He became a supporter of Pompey the Great, and as a member of the conservative and pro-aristocratic optimates faction, was elected consul in 63 BC. That year, he suppressed what came to be known as the Catiline Conspiracyto overthrow the government. He arrested and ordered the summary execution of its ringleaders captured in Rome.
A Wise Man Who Unwisely Underestimated a Precocious Teenager
In 60 BC, Cicero declined an invitation to join Caesar, Pompey, and Crassus in what became known as the First Triumvirate. He deemed the arrangement unconstitutional, and did all he could to undo it by driving wedges between the Triumvirs. Crassus was killed in battle in 53 BC, and Pompey and Caesar fell out eventually. The latter marched on Italy in 49 BC. Cicero sat out the civil war that followed, and spent that period in scholarly pursuits and writing books and treatises.
After Caesar’s assassination in 44 BC, Cicero promoted the cause of Gaius Octavius, Caesar’s adopted son and heir. The Roman philosopher and statesman sought to use the teenager as a cat’s paw in his conflict against Mark Antony, whom Cicero loathed and against whom he penned and orated scathing critiques known as The Philippics. Cicero quipped that he would “praise, raise, and erase” Octavius. He greatly underestimated the youth. As the future Augustus, Octavius would end the Roman Republic and replace it with the Roman Empire. As seen below, Cicero did not fare well in the new era.
Octavius shrewdly reconciled with Mark Antony and cut a deal that divided Rome between themselves and a third triumvir, Lepidus. They then proceeded to clean house, and eliminated all enemies and potential opponents of the regime in a bloody purg. Cicero, as an avowed enemy of Mark Antony, was on the new regime’s list of the proscribed. He fled Rome, but was captured and killed on December 7th, 43 BC. A vindictive Antony then had Cicero’s severed head and hands displayed in the rostra, or speaker’s platform in Rome’s Forum. That was after Antony’s equally vindictive wife had pierced the orator’s tongue with knitting needles.
Defeated politically, the Roman Republic’s most famous philosopher Cicero triumphed intellectually. His impact extended far beyond his own days. He left behind a trove of writings on philosophy, politics, rhetoric and oratory, whose rediscovery in the thirteenth century greatly interested scholars. Their numbers included Petrarch, who helped jump-start the Renaissance. The Roman Republic philosopher influenced European literature and ideas more than any single prose writer before or since. His impact on the Latin language was such that until the nineteenth century, all European prose could be viewed as being a return to or a reaction against Cicero’s style.
Heraclitus of Ephesus (535 – 475 BC) was an Ancient Greek philosopher who advanced the notion that the essence of the universe is constant change. To that end, he coined the phrase “no man ever steps into the same river twice“. It is based on the notion that everything, like the ever moving droplets of water drifting downstream on a river, is in constant motion and flux, even if the motion is not readily perceptible. He also propagated the notion of a “unity of opposites”, whereby the universe is a system of balanced exchanges in which all things are paired in a relationship with things that exhibit contrary properties.
A highly introspective man, Heraclitus did not come by his philosophy through learning at the hands of another philosopher, but was self-taught. Critical of other philosophers, Heraclitus had a dim view of humanity, loathed mobs and democracy, and preferred instead the rule of a few wise men. It was a concept that Plato later distilled into the notion that the ideal ruler would be a philosopher-king. Heraclitus thought wealth was a form of punishment, so he wished that his fellow Ephesians, whom he hated, be cursed with wealth as punishment for their sins.
In short, Heraclitus was a misanthrope, and his misanthropy led him to avoid contact with other people for long stretches. In those periods, he wandered alone through mountains and wilderness, surviving on plants and what he could scavenge. As Diogenes put it: “finally, [Heraclitus] became a hater of his kind, and roamed the mountains, surviving on grass and herbs“. He came to a bizarre end because of an affliction with dropsy, or edema – a painful accumulation of fluids beneath the skin and in the body’s cavities.
Doctors could offer neither cure nor relief, so Heraclitus, the self-taught philosopher, sought to apply his self-teaching skills to medicine and heal himself. He tried an innovative cure in which he covered himself in cow dung. The theory was that the warmth of the manure would dry and draw out of him the “noxious damp humor”, or the fluids accumulated beneath his skin. Heraclitus covered himself in cow poo, then lay out in the sun to dry. He was immobilized by the cow dung that dried around him into a body cast. He was thus unable to shoo off a pack of dogs that came upon and ate him alive.
As seen above, ancient Greek philosopher Socrates (470 – 390 BC) was a major troll. He was also the first moral philosopher of the Western ethical tradition, and is deemed a founder of Western philosophy. The widely accepted narrative is that he was an honest man who asked uncomfortable questions that his fellow Athenians did not like. So in a great miscarriage of justice, they railroaded, tried and executed him. At least that is how his most famous pupil, Plato, put it. However, if one digs into the context of what was going on in Athens at the time, that narrative loses its shine.
To many at the time, Socrates could have been viewed as a guru who taught some nasty people and filled their heads with anti-democracy views. His students then went on to do horrible things. There is no historic evidence that Socrates personally did any of the bad things done by his worst pupils. When called upon to personally participate in evil, he went home instead. However, when one considers how contemporary Athenians might have seen it, Socrates could be compared to a modern radical imam who might not personally get his hands dirty. However, his teachings still fire up others and inspire them to do awful things.
Socrates was a well-known and controversial figure in his native Athens. A gadfly, he often stopped people and asked them a series of questions that ultimately left them tied up in logical knots and contradicting themselves – the Socratic Method. That made him unpopular with many, and he was frequently mocked in the plays of comic dramatists, such as Aristophanes’ The Clouds. He emerged when Athens was at the height of her power – a flourishing democracy and the most powerful polis, or city state, of the era. A bit like the USA of the Greek world.
Socrates questioned democracy. That was music to the ears of Athens’ snobby rich young – think the equivalent of modern trust fund preppy spoiled brats. He validated their view that privileged people like them had a natural right to lord it over the unwashed masses. One of those students, Alcibiades, went on to betray Athens and turn it upside down and inside out in the Peloponnesian War, which ended catastrophically for the Athenians. Socrates was not responsible for the actions of Alcibiades, who was a live wire and dangerous force of nature. However, Alcibiades is an example of the kinds of privileged youth who liked Socrates because they thought he was “edgy”.
A Philosopher Whose Students Imposed a Bloody Reign of Terror
In and of itself, Socrates’ street trolling was an annoyance, but it did not anger his fellow citizens enough to want to kill him. Nor was the fact that he inspired and was liked by Ancient Athens’ version of preppy snobs sufficient to rile up other Athenians so much that they wanted Socrates dead. What took him from an irritant to a hated menace was the rise of the Thirty Tyrants – a cabal of rich Athenians who overthrew the democratic government. Their leader was Socrates’ student Critias, and their numbers included other pupils of the famous philosopher. They installed a collaborationist regime supported by Sparta, Athens’ longtime enemy which had defeated it after a decades-long Peloponnesian War.
The Thirty Tyrants’ government was an oligarchy dominated by aristocrats, and a bloodthirsty one at that. In its brief period of power, the regime carried a deadly purge against democratic supporters in which an estimated 5% of Athenian citizens were murdered. Others had their property confiscated and were forced to flee into exile. To put that in a modern context, picture if America’s 1%, led by radical devotees of Ayn Rand, carried a coup backed by China or Russia, and overthrew the US government. Then they installed a radical libertarian government, and rolled rights back to the days when only the propertied upper class got to vote. To cow the population into submission, they then sent out death squads that killed about sixteen million Americans – about 5% of the 2022 population.
Was This Philosopher Treated as Unfairly as People Have Been Led to Believe for Centuries?
Although Socrates’ students led the Thirty Tyrants, he refused to get his own hands dirty in their reign of terror. In one narrative, he was ordered to participate in the roundup and execution of some people. Instead, he heeded the dictates of his inner conscience and went home. Laudable as that might have been, to many Athenians it was not enough. When a popular uprising eventually overthrew the Thirty Tyrants and restored democracy, Socrates had a target on his back. To put it in a modern context, imagine if Americans rose up in revolt to overthrew a radical libertarian regime of Ayn Rand devotees that had slaughtered sixteen million of their fellow citizens, and restored democracy. If Ayn Rand was still alive, even if she had not personally killed anybody, she would probably not fare well.
That was the context in which Socrates was viewed by many Athenians after the Thirty Tyrants’ bloody regime. To many, the city’s most famous philosopher had fed the rich snobs’ sense of entitlement, and fed their resentment of jumped up commoners with a say in government. That philosophy inspired them to commit treason and cooperate with a foreign enemy to overthrow the government and slaughter said commoners. Seen from that perspective, that the Athenians afforded Socrates a trial – a fair and open one in which he got to defend himself, unlike those slaughtered by his Thirty Tyrant acolytes – demonstrated remarkable restraint. They could have simply dragged him out of his house and tore him limb from limb with their bare hands soon as democracy was restored.
The narrative that Socrates’ trial and execution were a grave miscarriage of justice was penned by Plato (427 – 347 BC). Socrates’ most famous student, Plato was a great philosopher in his own right, who went on to teach yet another great philosopher, Aristotle. That trio laid the foundations of Western philosophy and science. Plato ranks among history’s most influential figures, and for over two millennia has been one of the world’s most widely read and studied philosophers. In addition to his writings, he founded the Western World’s first institution of higher learning, The Academy in Athens.
The origins of Western political philosophy can be traced back to Plato’s writings, particularly the Laws and Republic. In addition to his impact on science, philosophy, politics, and education, Plato greatly influenced spirituality and religion. His impact on Christianity can be seen in the strong influence his philosophy exerted on Saint Augustine of Hippo, early Christianity’s most influential theologian. His writings, which drew quite a bit from Plato, played a key role in foundational Christian philosophy, and thus shaped that religion and subsequent Western thought.
Just About All We Know About Socrates is Derived from One of His Admiring Pupils
Among Plato’s innovations was the introduction of dialogue and dialectic forms in philosophical writings. In his books, characters engage in intellectual debates, in the course of which philosophic points are advanced, challenged, shot down, or honed. Nothing written by Socrates has survived. Virtually all we know of his philosophy has been transmitted through Plato, whose early writings are generally considered to be Plato’s account of Socrates’ life and thought. Plato’s later writings, such as his best known book, the Republic, are deemed to contain Plato’s own philosophy, as the main characters speak for Plato himself.
Plato’s sympathetic narrative about Socrates should be understood in the context of his background and political leanings. He was a philosopher born in a wealthy and conservative, even reactionary, family. Plato was related to two of the Thirty Tyrants who overthrew Athens’ democracy. That family influence is reflected in Plato’s political philosophy, which is skeptical of democracy and favors enlightened authoritarianism. When the Thirty Tyrants were overthrown and democracy was restored, a counter reaction set in against anti-democratic thought. It culminated in the execution of Plato’s teacher Socrates in 399 BC.
An Anti-Democratic Philosopher Who Suffered When Democracy Was Restored
From the perspective of pro-democracy Athenians, Socrates was not a harmless old philosopher who merely asked uncomfortable questions. Instead, he was a pernicious guru who taught a subversive philosophy that catered to aristocrats hostile to democracy. Many of Socrates’ students had committed treason Most infamous among them was Alcibiades. Athens lost that war, and Socrates’ acolytes overthrew the democratic government and replaced it with the with the Thirty Tyrants regime, which engaged in widespread murder.
When democracy was restored, people looked back at Athens’ glory days only three decades past, when their polis was at the height of its power and prosperity. They contrasted those days with their reduced circumstances in the aftermath of catastrophic defeat and violent repression, and asked themselves “what went wrong?“ Socrates and his boat rocking were among the answers. Athens became unhealthy for Socrates’ students, and Plato fled to travel around the Mediterranean. He returned years later, after passions had cooled, and founded The Academy in the 380s BC. It is in that context that Plato penned his sympathetic account of Socrates.
There Was Way More to Pythagoras Than “A Squared Plus B Squared Equals C Squared”
As seen in a previous entry, Pythagoras was not your everyday philosopher. It is unfortunate that the first thing that comes to people’s mind when they think of him – probably the only thing that comes to mind – is the Pythagorean theorem from grade school. There was so much more to the man than “for right angled triangles, side A squared plus side B squared equals side C (hypotenuse) squared“. The real Pythagoras was actually more of an eccentric – or even lunatic – cult leader, who was good at math and got his followers to worship numbers.
However, the theorems and equations competed with Pythagoras’ other weird beliefs. For example, he launched a crusade against beans: he equated eating them to cannibalism and to eating one’s parents. Pythagoras was not just a mathematician: he was a full blown cult leader. Those who followed him were not just people who liked math, but adherents of a full-blown religion that revolved around math. Pythagoras preached that the world was based and built on numbers. He taught his followers that reality and the entire universe were controlled by mathematical harmonies.
A Philosopher Who Believed That People Lost a Bit of Their Soul Whenever They Farted
Pythagoras also preached that math was holy, and that numbers were sacred and godlike. The number seven, for example, was associated with wisdom, and eight was associated with justice. Ten was the universe’s holiest number, and the Pythagoreans worshipped it with a prayer that began: “Bless us divine number, who created gods and men“. Their most sacred symbol was the Tetractys, a triangle with ten points across four rows. Pythagoras advanced many reasons to avoid beans, especially fava beans. One of the funnier ones was his belief that human beings lost a part of their soul whenever they farted, with a bit of their inner being exiting along with the expelled gasses.
More seriously, the influential mathematician and philosopher believed that beans contained the souls of the dead. He got there after a “scientific” experiment to prove that humans and beans were spawned from the same source. Pythagoras buried beans in mud, and left them for a few weeks before retrieving them. When he dug them up, he saw a resemblance to human fetuses. So he convinced himself of an intimate relationship between beans and humans, and reasoned that eating beans was akin to eating human flesh. Thus, Pythagoras equated eating beans with cannibalism, and not just as any cannibalism, but cannibalism of one’s father and mother. As he explained it to his followers: “Eating fava beans and gnawing on the heads of one’s parents are one and the same“.
In addition to his great dislike of beans, Pythagoras taught his followers that meat should never be eaten under any circumstances. That made this strange philosopher one of the earliest known vegetarians in Western civilization. His stance was based on a belief in the transmigration of souls – the notion that souls pass from one body to another, whether human or animal. As such, Pythagoras refrained from eating meat out of fear that he might end up accidentally eating a deceased friend or relative. The belief in the transmigration of souls also led Pythagoras to advocate for kindness towards animals.
In one instance, he came across a man beating a dog. He recognized in its yelps of pain the voice of a recently deceased friend. So he physically intervened and got the man to release the dog, and thus spared it a life of misery with a cruel owner. Members of Pythagoras’ cult worshipped him as a demigod, and referred to him as “the divine Pythagoras“. They believed that he possessed supernatural powers that allowed him to write words on the face of the moon. They also thought that Pythagoras could tame the wild animals of the earth and the birds of the sky by stroking them, and that he could control them with his voice.
Pythagoras’ followers claimed that he was the son of the god Apollo, or according to some sources, that he was fathered by the god Hermes. As a sign of his divinity, they claimed that he had a golden thigh, whose shimmering sight turned doubters into believers. Pythagoras encouraged such beliefs, and claimed that the gods had blessed him with the ability to return to life after death via a divine rebirth. He allegedly murdered his most famous acolyte, Hippasus, a genius in his own right, in a dispute over math. Pythagoras’ math religion revolved around the belief that numbers could explain all in life. Central to that was the belief that everything in the universe could be explained by rational numbers that could be expressed as fractions.
Then Hippasus demonstrated the existence of irrational numbers. It turned out to be a bad move on his part. For Pythagoras and his closest adherents, Hippasus’ irrational numbers were like a turd dropped into their punch bowl. Unfortunately for Hippasus, although a genius, he was not very smart. He demonstrated his irrational numbers while on a boat that contained only him, Pythagoras, and a bunch of other Pythagoreans. He wrestled Hippasus to the side of the boat, and dunked his head underwater until his struggling student ceased to breathe. The homicidal philosopher then tossed the corpse overboard, and warned his other followers to never mention what they had seen or heard.
Like most cults, that of Pythagoras and his followers alarmed the rest of the community in which they dwelt. At first, the people of Croton, where Pythagoras lived with his followers, put up with the math weirdos in their midst. Then the Pythagoreans stepped over the line. Overestimating their power – and the appeal of their beliefs – they made a bid for power, and tried to compel ordinary citizens to adopt the Pythagorean lifestyle. It did not end well for Pythagoras and his adherents.
The Pythagoreans morphed into an ancient Greek math ISIS or Boko Haram. They tried to prevent the people of Croton from eating beans, and directed that at all costs, they must not eat meat. The good people of Croton reacted violently, and it ended in a general persecution of Pythagoreans. By the time the dust had settled, many of the cultists had been killed, while the rest were forced to flee. The survivors attempted to regroup and carry on elsewhere, but they never again achieved as much prominence or power as they had secured in Croton, and they cult soon faded away.
As to Pythagoras himself, the mathematician and philosopher was killed in the backlash against his cult. A number of different accounts depict his end. One of them has it that, as he fled for his life with angry pursuers hot on his heels, Pythagoras flight took him to a field of beans. Beans were sacred to Pythagoras, so he stated that he would rather die than step on a single bean. That is what happened when his pursuers caught up with him at the edge of the bean field, and slit his throat.
In another version, the people of Croton attacked a house in which Pythagoras and his followers were conducting a meeting, and set it on fire. Pythagoras escaped with a small group of followers, and eventually took shelter in a temple. There, he was besieged, and eventually starved to death. He had refused to eat the only food available: beans. Contra his claims, Pythagoras was not a god, and contra his and his followers’ prediction, he did not return from the dead.
Where Did We Find This Stuff? Some Sources and Further Reading