Since the dawn of time, mankind has sought to understand both the meaning and the significance of its own existence. Although millennia of philosophers have all shared the same core mission and drive to answer these most fundamental questions about religion and philosophy, certain factors such as time and culture ultimately shaped their worldview—and their suppositions about humanity itself.

The most significant of the early philosophers came from three ancient empires: Greece, Rome, and China. These philosophers lived and studied between the years of 624 B.C. and 450 A.D., although most lived between 550 B.C. and 150 B.C. and all are generally grouped together as B.C. Philosophers. The earliest thinkers, who collectively formed the Milesian School in the ancient city of Miletius in Ionia along the Aegean Sea (now mostly contained within the modern nation of Turkey), devoted most of their study to contemplating the foundations of nature and cosmology. Because science had not advanced too far at the time, their hypotheses were quite literal, and the studies of Anaximenes, Thales, and Anaximander became less noted for their ideas and for more directly inspiring the later work of the Greeks.

Before the more classically grounded Greeks, however, Diogenes of Apollonia and Pherecydes of Syros published significant tomes questioning not just the findings of their peers, but postulating a subjective nature of existence; Diogenes in particular was reviled by his peers, although his work would tremendously influence that of Socrates, perhaps the most famous of the early philosophers—or of any era of philosophy.

Socrates deviated from his predecessors in that he never committed his ideas to paper, but instead let his students’ conclusions act as his legacy. As such, his specific views remain in doubt, although his list of students reads like a history book: Plato, Aristotle, and Critias, to name a few. But most significantly, Socrates’ manner of investigating theoretical, hypothetical, and moral inquires was so widely embraced it became his greatest legacy as the Socratic Method formed most of Western civilization’s manner of inquiry, be it for abstract discussion or for more concrete scientific pursuits.

The Romans of this era pursued similar lines of thought and existential exploration, although were more likely to answer questions through the prism of the empire, rather than of the world; as much of their work was funded by wealthy patrons in commerce and government, it became important for the great philosophers of the day to act not just as products of Rome, but also as extensions of it. Their contributions were not as great as those of the Greeks, but Augustine of Hippo (later sanctified as Saint Augustine) and Damascius became symbolc of the split between Eastern and Western civilization, as the former’s embrace of Christianity and the latter’s rejection of it led to their respective executions, illustrating the theological divide within the crumbling Roman Empire.

Ancient Chinese philosophy was generally of a much more practical and grounded nature; rather than looking to answer questions of mysticism and cosmology, it looked at more human concepts such as politics, class, morality and warfare. The most famous school of Chinese philosophy, Confucianism, was led by its namesake Confucius, not just the most influential of Chinese thinkers but perhaps the most influential of any philosopher. Confucius argued very strongly against artificial social structures based on might and wealth, and believed instead in a human meritocracy in which a person’s abilities and knowledge determined their standing in the world. Although his teachings were embraced widely, Chinese culture and government has repeatedly reinterpreted these teachings to reflect the mores and needs of the times.

Through the Dark Ages and Middle Ages, philosophy was relegated to a second-class academic pursuit as more secular schools of thought took precedence. However, 17th century philosophers such as Francis Bacon, John Locke, Isaac Netwon, and Galileo Galilei combined long-standing questions of human existence with more practical pursuits like politics and science.

The 18th century philosophers, however, ushered in the famously dubbed “Age of Enlightenment,” in which philosophical thought was applied on a full social scale, thanks to a preponderance of wealth throughout Europe, the discovery of the New World and, most importantly, the advent of the printing press. Great thinkers of the day could now spread their findings and ideas to a wider audience, although literacy rates were still low among general populations. The most famous thinkers of this period, in turn, were not strictly thinkers at all: Benjamin Franklin and Thomas Paine became famous as colonial liberators, Joseph Haydn made his name as a composer, and Adam Smith developed the foundations of the modern free market economy.

When the 19th century philosophers had their turn at prominence, their findings became more divisive and less unifying than their predecessors’. Immanuel Kant disrupted the entire idea of learning with his bridge between empiricism and rationalism, and Friederich Nietzche’s rejection of moral codes in general (and religious codes in particular) split higher thinking in two. Karl Marx’ postulations on a socialist society would lead to the second-greatest divide in the coming century between classes and competing governmental structures; the greatest divide, of course, belonged to Charles Darwin and his theory of evolution, which presented ideas on life which ran counter to the thousands of years’ worth of philosophy (and religion) that came before.

Although the centuries before them suggested great promise, the 20th century philosophers found themselves too splintered to put forth and great advancements in deeper thinking. What those philosophers managed to achieve, however, were tremendous specific advances in theoretical and applied sciences based on new ways of critical analysis stemming from the teachings of their predecessors. As existentialism grew in usefulness, it in turn drove much of the analysis of the history happening around it; great thinkers of the day, such as Jean-Paul Sartre and Simone de Beauvoir, increasingly found their work stemming not as standalone ideas but as results of larger inquiries. Once, philosophers were simply thinkers; with time, those philsophers had become doers, as well.