Student of Socrates, teacher of Aristotle, all-around legend, Plato (428/427 BCE—348/347 BCE) was arguably the greatest Greek philosopher of them all. According to The Encyclopedia Britannica, he was:
…best known as the author of philosophical works of unparalleled influence.
Building on the demonstration by Socrates that those regarded as experts in ethical matters did not have the understanding necessary for a good human life, Plato introduced the idea that their mistakes were due to their not engaging properly with a class of entities he called forms, chief examples of which were Justice, Beauty, and Equality. Whereas other thinkers—and Plato himself in certain passages—used the term without any precise technical force, Plato in the course of his career came to devote specialized attention to these entities. As he conceived them, they were accessible not to the senses but to the mind alone, and they were the most important constituents of reality, underlying the existence of the sensible world and giving it what intelligibility it has.