In his Theory of Forms, which he presents in his Socratic dialogues, Plato presents a solution to the problem of appearance and change, which in philosophy is also called the problem of universals.
The problem of universals is a logical and metaphysical question that has bugged many philosophers and thinkers of the past. In philosophy and metaphysics, the "universals" are entities which describe the properties and relations, as well as the characteristics and attributes of particular objects, that these objects share or have in common; for instance, a white cloud and a white flower both share the same color or same property—whiteness. The problem of appearance and change or the problem of universals, thus, asks the question whether or not these universals exist or have any meaning beyond the human mind; in other words, does the whiteness of the cloud and the flower exist as a separate entity? Two theories arise from this problem—realism and nominalism; realists argue that the universals exist in beyond the human mind, while the nominalists argue that the universals do not exist in the same way as physical entities do.
Plato's theory is realistic in context. He acknowledges the fact that the physical world is prone to change—everything changes its appearance and every object and subject in nature and the universe reflects change; however, he also points out that because the world is constantly changing, it become unreliable. Thus, he theorizes that the physical world is not, in fact, the "real" world, instead he claims the that a separate reality or realm exists beyond space and time, known as the Realm of Forms and Ideas. In this realm, the universals exists as absolute truths; they are permanent and unchangeable abstract forms and ideas, and as such they cannot be affected by the entities of the physical world.