Plato is arguably the single most influential philosopher of all time. So significant is his work that the modern British philosopher Albert North Whitehead quipped that "the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato." If Whitehead overstated his case, it was not by much. Plato's work extends to virtually every area of philosophy.

Political Philosophy and the Just City

Although Plato's writings extend to every corner of academia, political philosophy was his most common subject. In "Republic," perhaps the most famous of his dialogues, he tackled the difficult questions of what constitutes justice and how a just state should function. Plato's answers were controversial, and "Republic" has encouraged philosophical debate in every century after its writing. Later philosophers like Robert Nozick and John Rawls drew heavily on Plato's work in constructing their own conceptions of justice and the state.

Philosophy of Rhetoric

Plato also studied and wrote extensively on rhetoric, the art of persuasion. Two of Plato's most important dialogues, "Phaedrus" and "Gorgias," address questions about the nature of rhetoric, according to the Stanford Encyclopedia of Philosophy. For Plato, rhetoric and argument was a way to deduce truths about the world through careful introspection. However, he also saw rhetoric as a potentially dangerous weapon in the hands of sophists and demagogues. Later philosophers like Aristotle, Cicero, and Augustine expanded on Plato's philosophy of rhetoric.

Platonic Epistemology

Yet another of Plato's essential contribution to philosophy is his work on epistemology. Epistemology is the study of knowledge, or how people come to know things. According to the European Graduate School, Plato was among the first philosophers to consider the idea of a priori knowledge, defined as knowledge that exists independent of experience. In "Republic" and other dialogues, Plato argued that human experience was always limited and deceptive. The real truth, he said, emerged not from the real world but from the world of ideas. Plato argued that idealized "forms" represented the true, perfect version of every thing or idea in the universe. This powerful concept has puzzled and inspired great philosophers like David Hume, Martin Heidegger and Jacques Derrida for generations.

Plato's Dialectic

Beyond his writings and findings, Plato also contributed to philosophy a new method for answering philosophical questions. Nearly all of Plato's writings took the form of dialogues between Socrates and various other characters. The characters disagree and argue with each other. Plato's use of dialogue pitted arguments and ideas against each other, allowing the best ideas to rise to the surface. This dialectical method ensures rigorous scrutiny of every premise and conclusion. Although few modern philosophers write in dialogue, the dialectic has influenced subsequent methods of philosophical explication.