Rationalist Epistemology: Plato

Epistemology is the study of the nature, source, limits, and validity of knowledge.  It is especially interested in developing criteria for evaluating claims people make that they "know" something.  In particular, it considers questions such as: What is knowledge?  What is the difference between knowledge and opinion or belief?  If you know something, does that mean that you are certain about it?  Is knowledge really possible?

Traditionally, philosophers have thought that if someone (P) knows X, that means that

Despite the fact that intuition is a common phenomenon, philosophers have often been hesitant to identify it as a form of knowledge--primarily because there seems to be little way to determine whether it does, in fact, provide knowledge as opposed simply to lucky guesses.  So most philosophers focus, instead, on reason and sense experience as the bases of knowledge.  These two latter ways of approaching the question of knowledge are identified as rationalism and empiricism.

A rationalist epistemology claims that knowledge (as opposed to opinion) is possible only if it is based on self-evident and absolutely certain principles.  Such principles are not learned through experience; instead, they are implicit in the very notion of reasoning (in Latin: ratio) itself.  Sense experience cannot provide the certainty needed to guarantee that what we claim to know is true.  So, like mathematicians, we have to rely on reason itself as the basis for determining whether our opinions are justified true beliefs (that is, knowledge).

Knowledge for the rationalist is what can be deduced from principles that undoubtable ("indubitable") or are true by definition. Examples of such principles include: "A whole is always greater than any one of its parts," "A thing cannot be and not be at the same time in the same respect," "Bachelors are unmarried males," "Unicorns are imaginary animals,"and "Triangles have three sides." These statements are known with certainty to be true because the very meaning of the terms involved (e.g., wholes, parts, things, bachelors, unicorns, triangles) requires that some judgments we make about them do not rely on sense experience.  We thus know about such things prior to any sense experience we have or could have about them.  This knowledge is called a priori.  Any knowledge that relies on (that is, comes after or is posterior to) sense experience is called a posteriori.

Plato is an example of a rationalist. He says 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 often unreliable. It can be corrected and evaluated for dependability only by appealing to principles that themselves do not change. These unchanging principles (or "Forms") are the bases of what it means to think or reason in the first place.  So 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 (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 (i.e., 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 (the visible world) is merely a fleeting image of the intelligible world--what Plato calls the realm of the Forms. Physical objects are real only insofar as they are intelligible, but they can be intelligible only in terms of that which does not change. What makes a thing intelligible as a certain kind of thing cannot be constantly changing: otherwise, it could not be identified as that kind of thing, nor would it be that kind of thing. 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 (non-sensual) realm in terms of which physical things are said to exist intelligibly. That is the realm of the Forms.

Plato's simile of the sun, image of the divided line, and allegory of the cave are intended to clarify exactly how the things we experience in the sensible, ordinary world (e.g., chairs, drawn triangles) are less real than the ideal models (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 things 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, mathematical concepts such as triangularity--are more real than, say, triangular blocks of 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 changing.
Epistemology Ontology Source of Being and Intelligibility
Pure reason
(grasped mathematically)
The Forms
The Intelligible World The Good
Understanding (subsuming the particular under the general)
Opinion (conjecture) 
Belief, sense experience
Particular sensible objects
The Visible World The Sun
Imagination, Hearsay Images, Shadows, Reflections

Some Forms (e.g., chair-ness) are the ideal models in terms of which physical objects (e.g., chairs) exist and are intelligible. Other even higher Forms (e.g., equality, 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 chair-ness 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 a number or 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.

Meno's Paradox and the Immortality of Soul: how will you know what you are looking for if you first don't already know it (and thus have no reason to go looking for it)?  But why look for something you already have?  This is the paradox raised in Plato's dialogue called the Meno.  In answer to "Meno's Paradox," Plato suggests that before we were born we existed in another realm of being (the realm of the Forms). The shock of being born makes us forget what we knew in that realm. But when we are asked the right questions or have certain experiences, we remember or "recollect" innate (inborn) truths. So if we existed before our births, there is every reason to think that we will continue to exist after our deaths.