20 Major Philosophers & Their Big Ideas

Philosophy is complicated stuff. It’s the search for meaning, for greater understanding, for answers to the questions surrounding our existence, our purpose, and the universe itself. So obviously, attempting to sum it up in a few pithy blurbs is a fool’s errand. Well, consider us your fool, because that’s exactly what we’ve set out to do.

We’re certain Socrates would question our motives, Emerson would criticize us for writing on the subject so far removed from nature, and Nietzsche would make fun of us until we cried. But we think it’s worth the risk to give you a quick shot of knowledge while you prepare for your exam, tighten up your essay, or begin the research process.

Because philosophy is such a broad and encompassing subject — I mean, it’s basically about everything — we don’t claim to cover the subject comprehensively. Honestly, the only real way you can fully comprehend the theories, epistemologies, and frameworks described here is to read the writing created by — and critique dedicated to — each of these thinkers. But what follows is your introduction, a rapid-fire look at 20 Major Philosophers, their Big Ideas, and their most important written works. But think fast, because these mindblowers come at a furious pace.

1. Saint Thomas Aquinas (1225–1274)

Thomas Aquinas was a 13th century Dominican friar, theologian and Doctor of the Church, born in what is known today as the Lazio region of Italy. His most important contribution to Western thought is the concept of natural theology (sometimes referred to as Thomism in tribute to his influence). This belief system holds that the existence of God is verified through reason and rational explanation, as opposed to through scripture or religious experience. This ontological approach is among the central premises underpinning modern Catholic philosophy and liturgy. His writings, and Aquinas himself, are still considered among the preeminent models for Catholic priesthood. His ideas also remain central to theological debate, discourse, and modes of worship.

Aquinas’ Big Ideas

Aquinas’ Key Works

2. Aristotle (384–322 BCE)

Aristotle is among the most important and influential thinkers and teachers in human history, often considered — alongside his mentor, Plato — to be a father of Western Philosophy.” Born in the northern part of ancient Greece, his writings and ideas on metaphysics, ethics, knowledge, and methodological inquiry are at the very root of human thought. Most philosophers who followed — both those who echoed and those who opposed his ideas — owed a direct debt to his wide-ranging influence. Aristotle’s enormous impact was a consequence both of the breadth of his writing and his personal reach during his lifetime.

In addition to being a philosopher, Aristotle was also a scientist, which led him to consider an enormous array of topics, and largely through the view that all concepts and knowledge are ultimately based on perception. A small sampling of topics covered in Aristotle’s writing includes physics, biology, psychology, linguistics, logic, ethics, rhetoric, politics, government, music, theatre, poetry, and metaphysics. He was also in a unique position to prevail directly over thinking throughout the known world, tutoring a young Alexander the Great at the request of the future conqueror’s father, Phillip II of Macedon. This position of influence gave Aristotle the means to establish the library at Lyceum, where he produced hundreds of writings on papyrus scrolls. And of course, it also gave him direct sway over the mind of a man who would one day command an empire stretching from Greece to northwestern India. The result was an enormous sphere of influence for Aristotle’s ideas, one that only began to be challenged by Renaissance thinkers nearly 2,000 years later.

Aristotle’s Big Ideas

Aristotle’s Key Works

3. Confucius (551–479 BCE)

Chinese teacher, writer, and philosopher Confucius viewed himself as a channel for the theological ideas and values of the imperial dynasties that came before him. With an emphasis on family and social harmony, Confucius advocated for a way of life that reflected a spiritual and religious tradition, but which was also distinctly humanist and even secularist. Confucius — thought to be a contemporary of Taoist progenitor Lao-Tzu — had a profound impact on the development of Eastern legal customs and the emergence of a scholarly ruling class. Confucianism would engage in historic push-pull with the philosophies of Buddhism and Taoism, experiencing ebbs and flows in influence, its high points coming during the Han (206 BCE–220 CE), Tang (618–907 CE), and Song (960–1296 CE) Dynasties. As Buddhism became the dominant spiritual force in China, Confucianism declined in practice. However, it remains a foundational philosophy underlying Asian and Chinese attitudes toward scholarly, legal, and professional pursuits.

Confucius’ Big Ideas

Confucius’ Key Works

4. René Descartes (1596–1650)

A French philosopher, mathematician, and scientist, Descartes was born in France but spent 20 years of his life in the Dutch Republic. As a member of the Dutch States Army, then as the Prince of Orange and subsequently as Stadtholder (a position of national leadership in the Dutch Republic), Descartes wielded considerable intellectual influence over the period known as the Dutch Golden Age. He often distinguished himself by refuting or attempting to undo the ideas of those that came before him.

Descartes’ Big Ideas

Descartes’ Key Works

5. Ralph Waldo Emerson (1803 82)

A Boston-born writer, philosopher, and poet, Ralph Waldo Emerson is the father of the transcendentalist movement. This was a distinctly American philosophical orientation that rejected the pressures imposed by society, materialism, and organized religion in favor of the ideals of individualism, freedom, and a personal emphasis on the soul’s relationship with the surrounding natural world. Though not explicitly a “naturalist” himself, Emerson’s ideals were taken up by this 20th century movement. He was also seen as a key figure in the American romantic movement.

Emerson’s Big Ideas

Emerson’s Key Works

6. Michel Foucault (1926-1984)

Historian, social theorist, and philosopher Michel Foucault, born in the riverfront city of Poiltiers, France, dedicated much of his teaching and writing to the examination of power and knowledge and their connection to social control. Though often identified as a postmodernist, Foucault preferred to think of himself as a critic of modernity. His service as an international diplomat on behalf of France also influenced his understanding of social constructs throughout history and how they have served to enforce racial, religious, and sexual inequality. His ideals have been particularly embraced by progressive movements, and he allied with many during his lifetime. Active in movements against racism, human rights abuses, prisoner abuses, and marginalization of the mentally ill, he is often cited as a major influence in movements for social justice, human rights, and feminism. More broadly speaking, his examination of power and social control has had a direct influence on the studies of sociology, communications, and political science.

Foucault’s Big Ideas

Foucault’s Key Works

7. David Hume (1711–77)

A Scottish-born historian, economist, and philosopher, Hume is often grouped with thinkers such as John Locke, Thomas Hobbes, and Sir Francis Bacon as part of a movement called British Empiricism. He was focused on creating a “naturalistic science of man” that delves into the psychological conditions defining human nature. In contrast to rationalists such as Descartes, Hume was preoccupied with the way that passions (as opposed to reason) govern human behavior. This, Hume argued, predisposed human beings to knowledge founded not on the existence of certain absolutes but on personal experience. As a consequence of these ideas, Hume would be among the first major thinkers to refute dogmatic religious and moral ideals in favor of a more sentimentalist approach to human nature. His belief system would help to inform the future movements of utilitarianism and logical positivism, and would have a profound impact on scientific and theological discourse thereafter.

Hume’s Big Ideas

Hume’s Key Works

8. Immanuel Kant (1724–1804)

Prussian-born (and therefore identified as a German philosopher), Kant is considered among the most essential figures in modern philosophy, an advocate of reason as the source for morality, and a thinker whose ideas continue to permeate ethical, epistemological, and political debate. What perhaps most distinguishes Kant is his innate desire to find a synthesis between rationalists like Descartes and empiricists like Hume, to decipher a middle ground that defers to human experience without descending into skepticism. To his own way of thinking, Kant was pointing a way forward by resolving a central philosophical impasse.

Kant’s Big Ideas

Kant’s Key Works

9. Søren Kierkegaard (1813–55)

A Danish theologian, social critic, and philosopher, Kierkegaard is viewed by many as the most important existentialist philosopher. His work dealt largely with the idea of the single individual. His thinking tended to prioritize concrete reality over abstract thought. Within this construct, he viewed personal choice and commitment as preeminent. This orientation played a major part in his theology as well. He focused on the importance of the individual’s subjective relationship with God, and his work addressed the themes of faith, Christian love, and human emotion. Because Kierkegaard’s work was at first only available in Danish, it was only after his work was translated that his ideas proliferated widely throughout Western Europe. This proliferation was a major force in helping existentialism take root in the 20th century.

Kierkegaard’s Big Ideas

Kierkegaard’s Key Works

10. Lao-Tzu (also Laozi, lived between the 6th and 4th century BCE)

Historians differ on exactly when Lao-Tzu lived and taught, but it’s largely held that some time between the 6th and 4th centuries BCE, the “old master” founded philosophical Taoism. Viewed as a divine figure in traditional Chinese religions, his ideas and writings would form one of the major pillars (alongside Confucius and the Buddha) for Eastern thought. Lao-Tzu espoused an ideal life lived through the Dao or Tao (roughly translated as “the way”). As such, Taoism is equally rooted in religion and philosophy. In traditional telling, though Lao-Tzu never opened a formal school, he worked as an archivist for the royal court of Zhou Dynasty. This gave him access to an extensive body of writing and artifacts, which he synthesized into his own poetry and prose. As a result of his writing, his influence spread widely during his lifetime. In fact, one version of his biography implies he may well have been a direct mentor to the Buddha (or, in some versions, was the Buddha himself). There are lot of colorful narratives surrounding Lao-Tzu, some of which are almost certainly myth. In fact, there are some historians who even question whether or not Lao-Tzu was a real person. Historical accounts differ on who he was, exactly when he lived and which works he contributed to the canon of Taoism. However, in most traditional tellings, Lao-Tzu was the living embodiment of the philosophy known as Taoism and author of its primary text, the Tao Te Ching.

Lao-Tzu’s Big Ideas

Lao-Tzu’s Key Works

11. John Locke (1632–1704)

An English physicist and philosopher, John Locke was a prominent thinker during the Enlightenment period. Part of the movement of British Empiricism alongside fellow countrymen David Hume, Thomas Hobbes, and Sir Francis Bacon, Locke is regarded as an important contributor to the development of the social contract theory and is sometimes identified as the father of liberalism. Indeed, his discourses on identity, the self, and the impact of sensory experience would be essential revelations to many Enlightenment thinkers and, consequently, to real revolutionaries. His philosophy is said to have figured prominently into the formulation of the Declaration of Independence that initiated America’s war for independence from the British.

Locke’s Big Ideas

Locke’s Key Works

12. Niccolo Machiavelli (1469–1527)

Niccolo di Bernardo dei Machiavelli is at once among the most influential and widely debated of history’s thinkers. A writer, public office-holder, and philosopher of Renaissance Italy, Machiavelli both participated in and wrote prominently on political matters, to the extent that he has even been identified by some as the father of modern political science. He is also seen as a proponent of deeply questionable — some would argue downright evil — values and ideas. Machiavelli was an empiricist who used experience and historical fact to inform his beliefs, a disposition which allowed him to divorce politics not just from theology but from morality as well. His most prominent works described the parameters of effective rulership, in which he seems to advocate for leadership by any means which retain power, including deceit, murder, and oppression. While it is sometimes noted in his defense that Machiavelli himself did not live according to these principles, this “Machiavellian” philosophy is often seen as a template for tyranny and dictatorship, even in the present day.

Machiavelli’s Big Ideas

Machiavelli’s Key Works

13. Karl Marx (1818–83)

A German-born economist, political theorist, and philosopher, Karl Marx wrote some of the most revolutionary philosophical content ever produced. Indeed, so pertinent was his writing to the human condition during his lifetime, he was exiled from his native country. This event would, however, also make it possible for his most important ideas to find a popular audience. Upon arriving in London, Marx took up work with fellow German Friedrich Engels. Together, they devised an assessment of class, society, and power dynamics that revealed deep inequalities, and exposed the economic prerogatives for state-sponsored violence, oppression, and war. Marx predicted that the inequalities and violence inherent in capitalism would ultimately lead to its collapse. From its ashes would rise a new socialist system, a classless society where all participants (as opposed to just wealthy private owners) have access to the means for production. What made the Marxist system of thought so impactful though was its innate call to action, couched in Marx’s advocacy for a working class revolution aimed at overthrowing an unequal system. The philosophy underlying Marxism, and his revolutionary fervor, would ripple throughout the world, ultimately transforming entire spheres of thought in places like Soviet Russia, Eastern Europe, and Red China. In many ways, Karl Marx presided over a philosophical revolution that continues in the present day in myriad forms of communism, socialism, socialized democracy, and grassroots political organization.

Marx’s Big Ideas

Marx’s Key Works

14. John Stuart Mill (1806–73)

British economist, public servant, and philosopher John Stuart Mill is considered a linchpin of modern social and political theory. He contributed a critical body of work to the school of thought called liberalism, an ideology founding on the extension of individual liberties and economic freedoms. As such, Mill himself advocated strongly for the preserving of individual rights and called for limitations to the power and authority of the state over the individual. Mill was also a proponent of utilitarianism, which holds that the best action is one that maximizes utility, or stated more simply, one that provide the greatest benefit to all. This and other ideas found in Mill’s works have been essential to providing rhetorical basis for social justice, anti-poverty, and human rights movements. For his own part, as a member of Parliament, Mill became the first office-holding Briton to advocate for the right of women to vote.

Mill’s Big Ideas

Mill’s Key Works

15. Friedrich Nietzsche (1844–1900)

Friedrich Nietzsche was a poet, cultural critic, and philosopher, as well as possessor of among the most gifted minds in human history. The German thinker’s system of ideas would have a profound impact on the Western World, contributing deeply to intellectual discourse both during and after his life. Writing on an enormous breadth of subjects, from history, religion and science to art, culture and the tragedies of Greek and Roman Antiquity, Nietzsche wrote with savage wit and a love of irony. He used these forces to pen deconstructive examinations of truth, Christian morality, and the impact of social constructs on our formulation of moral values. Also essential to Nietzshe’s writing is articulation of the crisis of nihilism, the basic idea that all things lack meaning, including life itself. This idea in particular would remain an important component of the existentialist and surrealist movements that followed.

Nietzsche’s Big Ideas

Nietzsche’s Key Works

16. Plato (428/427?–348/347? BCE)

Greek philosopher and teacher Plato did nothing less than found the first institution of higher learning in the Western World, establishing the Academy of Athens and cementing his own status as the most important figure in the development of western philosophical tradition. As the pupil of Socrates and the mentor to Aristotle, Plato is the connecting figure in what might be termed the great triumvirate of Greek thought in both philosophy and science. A quote by British philosopher Alfred North Whitehead sums up the enormity of his influence, noting “the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato.” Indeed, it could be argued that Plato founded political philosophy, introducing both the dialectic and dialogic forms of writing as ways to explore various areas of thought. (Often, in his dialogues, he employed his mentor Socrates as the vessel for his own thoughts and ideas.) While he was not the first individual to partake of the activity of philosophy, he was perhaps the first to truly define what it meant, to articulate its purpose, and to reveal how it could be applied with scientific rigor. This orientation provided a newly concreted framework for considering questions of ethics, politics, knowledge, and theology. Such is to say that it is nearly impossible to sum up the impact of Plato’s ideas on science, ethics, mathematics, or the evolution of thought itself other than to say it has been total, permeating, and inexorable from the tradition of rigorous thinking itself.

Plato’s Big Ideas

Plato’s Key Works

17. Jean-Jacques Rousseau (1712–78)

Rousseau was a writer, philosopher, and — unique among entrants on this list — a composer of operas and classical compositions. Born in Geneva, then a city-state in the Swiss Confederacy, Rousseau would be one of the most consequential thinkers of the Enlightenment era. His ideas on human morality, inequality, and most importantly, on the right to rule, would have an enormous and definable impact not just on thinking in Europe, but on the actual power dynamics within Western Civilization. Indeed, his most important works would identify personal property as the root to inequality and would refute the premise that monarchies are divinely appointed to rule. Rousseau proposed the earth-shattering idea that only the people have a true right to rule. These ideas fomented the French Revolution, and more broadly, helped bring an end to a centuries-old entanglement between Church, Crown, and Country. Rousseau may be credited for providing a basic framework for classical republicanism, a form of government centered around the ideas of civil society, citizenship, and mixed governance.

Rousseau’s Big Ideas

Rousseau’s Key Works

18. Jean-Paul Sartre (1905–80)

A French novelist, activist, and philosopher, Sartre was a leading exponent of the 20th century existentialist movement as well as a vocal proponent of Marxism and socialism. He advocated for resistance to oppressive social constructs and argued for the importance of achieving an authentic way of being. His writing coincided with, and contrasted, the sweep of fascism through Europe, the rise of authoritarian regimes, and the spread of Nazism. Sartre’s ideas took on increased importance during this time, as did his actions. Sartre became active in the socialist resistance, which aimed its activities at French Nazi collaborators. Of note, one of his activist collaborators was both a romantic partner and a fellow major cohort of existentialism, Simone de Beauvoir. Following the war, Sartre’s writing and political engagement centered on efforts at anticolonialism, including involvement in the resistance to French colonization of Algeria. In fact, his involvement earned Sartre two near-miss bomb attacks at the hands of French paramilitary forces. Also notable, Sartre was supportive of the Soviet Union throughout his lifetime. Though occasionally serving to raise issues regarding human rights abuses as an outside observer, he praised the Soviet Union’s attempt at manifesting Marxism.

Sartre’s Big Ideas

Sartre’s Key Works

19. Socrates (470–399 BCE)

A necessary inclusion by virtue of his role as, essentially, the founder of Western Philosophy, Socrates is nonetheless unique among entrants on this list for having produced no written works reflecting his key ideas or principles. Thus, the body of his thoughts and ideas is left to be deciphered through the works of his two most prominent students, Plato and Xenophon, as well as to the legions of historians and critics who have written on him since. The classical Greek thinker is best known through Plato’s dialogues, which reveal a key contributor to the fields of ethics and education. And because Socrates is best known as a teacher of thought and insight, it is perhaps appropriate that his most widely recognized contribution is a way of approaching education that remains fundamentally relevant even today. The so-called Socratic Method, which involves the use of of questioning and discourse to promote open dialogue on complex topics and to lead pupils to their own insights, is on particular display in the Platonic dialogues. His inquisitive approach also positioned him as a central social and moral critic of the Athenian leadership, which ultimately led to his trial and execution for corrupting the minds of young Athenians.

Socrates’ Big Ideas

Socrates’ Key Works

20. Ludwig Wittgenstein (1889–1951)

Born in Austria to a wealthy family, Wittgenstein is one of philosophy’s more colorful and unusual characters. He lived a life of eccentricity and professional nomadism, dabbling in academia, military service, education, and even as a hospital orderly. Moreover, during his life, he wrote voluminously but published only a single manuscript. And yet, he was recognized by his contemporaries as a genius. The posthumous publication of his many volumes confirmed this view for future generations, ultimately rendering Wittgenstein a towering figure in the areas of logic, semantics, and the philosophy of mind. His investigations of linguistics and psychology would prove particularly revelatory, offering a distinctive window through which to newly understand the nature of meaning and the limits of human conception.

Wittgenstein’s Big Ideas

Wittgenstein’s Key Works

We hope this was enlightening for you. If it doesn’t help you ace your exam, it should at least give you plenty to think about. By all means, go ponder the universe, yourself, and that frail, fickle thing we call the human condition.

And because 20 philosophers are indeed just a small sampling from the whole history of human thought, stay tuned for another round of influential thinkers in the not-too-distant future. Let us know if there are any you’d like to see covered.