Plato is an example of a rationalist. Plato argued that sense experience fails to provide us with any guarantee that what we experience is, in fact, true. The information we get by relying on sense experience is constantly changing and is therefore unreliable. Such knowledge can be corrected and evaluated for dependability only by appealing to principles that do not change. These unchanging principles or "Forms" are the bases of what it means to think or reason in the first place. If we can show that an opinion or belief we have is based on these undoubtable principles of thought, we have a firm foundation for the opinion. That foundation is what allows us to think of a belief as more than simply opinion; it is what allows us to identify the belief as justified and true, and that is what is meant by knowledge
In short, in order to have knowledge or justified true belief, we have to transcend the ever changing flux of the physical world and grasp a permanent rational order behind the flux, an order that will demonstrate the universal in the particular. This "grasping" is an intellectual act of the mind, which, in its purest manifestation, is exclusively formal or mathematical. Such an intellectual act can take place only if there are certain innate ideas upon which it can be based. "Knowing", then, is an act of making the observable world intelligible by showing how it is related to an eternal order of intelligible truths.
In other words, the world of changing, material objects or the visible world is merely a fleeting image of the intelligible world. This is what Plato called the realm of the Forms. Physical objects are real only insofar as they are intelligible only in terms of that which does not change. So a thing is what it is in virtue of something that is not changing. But since the visible world is constantly changing, it cannot be used as the basis for identifying what things are. There must be an intelligible or non-sensual realm in terms of which physical things are said to exist intelligibly. That, according to Plato, is the realm of Forms.
Plato's simile of the sun, image of the divided line and allegory of the care are intended to clarify exactly how the things we experience in the sensible, ordinary world, things such as chairs, drawn triangles, are less real than the ideal models or Forms on which they rely for their existence and in terms of which they are intelligible. Just as drawings, reflections, or copies of sensible objects are not as real as the sensible thing on which they depend, so sensible things are not as real as the concepts in terms of which they are identifiable.
Concepts that rely on sensual imagination for their intelligibility, for example things such as mathematical concepts such as triangularity, are more real than, say triangular blocks or wood or drawings of triangles. But even though concepts that are based on sense experience are not limited to any particular expression and are unchanging, they are not as real as the Forms, which do not rely for their existence or intelligibility on anything sensual and unchanging.
Some Forms, such as chairness, are the ideal models in terms of which physical objects, such as chairs, exist and are intelligible. Other even higher Forms such as equality or justice provide the means by which not only physical objects, but also activities, relations, and even lower Forms themselves are identifiable. The Forms are not abstractions or generalizations based on our sensual experience of physical objects; rather, we know physical objects as what they are by knowing them in terms of their Forms.
As such, in order to know that a chair is a chair, we have to know what chairness is first, and that means that we cannot begin with sensible experience. Likewise, in order to know that two numbers are equal, or that an action is a just action, we have to know first what equality or justice is. But that already assumes we know what what an action is; and that can only be known by appealing to lower Forms that rely for their intelligibility and existence on higher Forms. The highest Forms are themselves intelligible and exist ultimately in terms of the "super" Form, the Good.
Thus, with Plato we have a much more full blown beginning to Rationalistic Epistemology.
