The philosophy of Plato, who lived in Greece from approximately 428 to 348 BC, is enormously important and influential in the history of Western thought. Some of the most prominent elements in the philosophy of Plato include ideas about the nature of moral virtue, theories of the best form of government, and Plato's theory of the forms. Identifying Plato's own beliefs is complicated by the fact that Plato wrote primarily in the form of dialogues and Plato himself never appears as a character in any of them. Rather than simply laying out a set of arguments, Plato presented his ideas in his writing by portraying conversations between two or more people, in which various ideas, arguments, and counter-arguments would be presented. One of the characters in these dialogues is usually Plato's teacher Socrates, who was a prominent and influential figure but left no written works of his own.
Later philosophers and scholars have sometimes disagreed about which of the ideas that appear in Plato's dialogues are Plato's own beliefs, presented through Socrates as a literary device, and which are beliefs held by the historical Socrates and reported but not necessarily endorsed by Plato. There are also disagreements as to whether some of the ideas in Plato's works, such as the description of an ideal city ruled by philosopher-kings in The Republic, were meant literally.
One well-known aspect of the philosophy of Plato is the idea of forms, which Plato proposed as an explanation of the nature of universals. A universal is a characteristic that can be present in multiple particular objects at the same time. For example, a red fire hydrant, red blood, and a red bird are particular objects that share the quality of redness, which is a universal. One common philosophical question, called the problem of universals, is whether universals are real entities and what their nature is if they are.
Plato believed that universals do exist and have an existence beyond the particular objects that happen to possess them, a position often called Platonic realism. It is contrasted with the belief called nominalism that only particular objects exist and the belief that universals do exist as real entities, but that their existence depends on the existence of particular objects that have them, a position commonly called Aristotelian realism.
To explain the nature of universals, Plato proposed the idea of abstract objects called forms that are the perfect, unchanging essences of all particular, concrete things. For example, all individual horses are instantiations of, or participate in, the form of the horse, and so they all share the same nature as horses despite being unique individuals that differ from each other in various other ways. Similarly, all red objects are reflections of the form of redness, all spherical objects reflections of the form of the sphere, and so on. This is true not only of physical characteristics, but also of more abstract concepts. Beautiful objects reflect the form of the beautiful, just actions reflect the form of the just, and so on.
The forms exists outside of time and space and can be understood only through reason rather than sensory observation. Despite having no physical existence, the forms in Plato's philosophy are in an ultimate sense more real than particular objects, because each trait of each particular object is a reflection of the forms. Plato's most famous expression of this idea appears in The Republic, in which he compares the world we perceive to shadows cast on the wall of a cave by solid objects moving in front of a fire, shadows that most people, unaware of the forms that underlie everything, mistake for reality.
The philosophy of Plato included his ideas on a great number of other topics, including ethics, human nature, and the nature and purpose of human activities such as art and rhetoric. In his dialogue The Republic, Plato proposed an analogy between the best form of government and the best ordering of the soul of the individual. He divided the individual soul into three parts or faculties: reason, the appetites or desires, and the spirit, which encompassed things like courage and willpower.
Plato believed that a just person was one governed by reason, with the appetites and spirit subordinate to it. Similarly, he argued, the most just state was one governed by a small elite composed of those who were most governed by reason and wisdom who ruled over those governed by appetites or spirit. This is commonly referred to as the concept of the philosopher-king. It is often disputed, however, whether Plato meant to advocate this as an actual model or real government or was only using it as a metaphor to describe his ideas about the nature of a virtuous person.