Plato was a philosopher and scholar who lived between 427 and 347 BC. He was a student of Socrates and later formed the first known "university," called the Academy. His best known work is The Republic, and his best known concept is the Theory of Forms.
The Theory of Forms states that, while experience is changing and illusory, ideal forms are unchanging and real. Plato advanced Parmenides' theory that both experience and forms are real. Aristotle and Socrates also began their philosophical thought from Parmenides, who was known as Parmenides of Elea and lived between 510 and 440 BC. Although his reasoning was shown by Aristotle, Socrates, Plato, and other philosophers to be mostly unsound, Parmenides, rather ironically, began the entire concept of logical deduction that was to make these scholars well-known.
Plato's theory holds that some stable, unchanging aspects of something must be a part of a person's sense experience for that thing to be considered real. For example, while there are many individual dogs, they are all made in the dog form. A form could be considered a pattern, a blueprint, or a design. People can recognize a form called a "dog" underneath what they experience as a "Great Dane," a "Yorkshire Terrier," a "Dalmatian," or a dog of mixed, unrecognizable breeds.
The Christian religion bases much of its theology on the influence of this theory. Man is believed to be made in God's image, for example, and while there are many individual men, they are believed to be universally made in God's form. The philosopher's deep influence on Christianity is one of the reasons the evolutionary philosopher A.N. Whitehead said "The safest characterization of Western philosophy is that of a series of footnotes to Plato."
Plato also thought form was universal, or ideal, in abstract concepts, such as in mathematical number classification, and the acknowledgment of the existence of abstract objects is called Platonism. Gottlob Frege and other logicians advanced this view of numbers as mathematical abstract objects through their theories of mathematical philosophy.
The Republic concerns the Theory of Forms in relation to the abstract concepts of justice and truth. Plato saw the ideal republic as a highly controlled state where individuals act to benefit society as a whole. Guardians, an elite class born and trained in their role, rule a society made up of common people as well as soldiers. The philosopher saw his vision of the republic as the ideal form, despite much criticism that this type of society could not be a viable one, even in ancient Greece.