The Epistemological Role of the Forms:

Epistemologically the Forms appear as the "objects" of knowledge. Knowledge is always "knowledge of..."; or in other words knowledge must have an "object" which it is knowledge of. Knowing what that object is, is essential for an adequate theory of knowledge, an epistemology. If we catch hold of the wrong object, a false illusion, and mistake it to be the real thing, we are in a state of ignorance. Hence the object of knowledge must be what is really so, not what merely appears to be the case. So Plato lays down the crucial claim that the object of knowledge is "real."

This is the epistemological role that the Form, is called upon to play; by definition:

A Form is the real object of knowledge.

From the fact that genuine knowledge is infallible, in distinction from fallible opinion, it follows that what is known cannot possibly be false.  From this conlusion it follows that knowledge is necessarily true or in other words, what is genuinely known cannot be otherwise than true.

Now from this conclusion that a belief is justified only if it is shown to be necessarily true, Plato deduces the somewhat startling conclusion that if the object of knowledge were to change, what is true of it at one moment might be false at another, but since knowledge is necessarily true, this cannot happen.  Thus the object of knowledge cannot change.

From this conclusion Plato draws the further inference that since "perishing" is a kind of change, it follows that the object of knowledge cannot perish.  It is therefore "immortal" (in Greek: "deathless") or eternal.  Within the context of Greek conceptions, "eternality" implied "divinity."

So the Forms are often spoken of as "divine" by which Plato's audience understood him to mean eternal and changeless.

But this conclusion implies that  physical objects we perceive with our senses (i.e. those things which are in space and time) cannot serve as the objects of knowledge, for what appears, for example, beautiful from one point of view, appears ugly from another; what is beautiful today, may grow old and decay and loose its beauty tomorrow. Thus the true lover of beauty should not pursue beautiful things, (i.e. changeable physical things which are for the moment beautiful) but should seek that which makes the beautiful things beautiful, what all beautiful things "share in" to some degree or another and which makes them beautiful. This is the essence of beauty, it is the "object" the lover of genuine beauty seeks, it is the Platonic "Form" of "Beauty Itself" or as Plato often puts it, "The Beautiful." Beautiful things will change and perish and cease to be beautiful, but Beauty Itself, the Form of Beauty will forever be what-it-is-to-be-beautiful.

When the lover of knowledge comes to have knowledge of Beauty Itself, his mind grasps the essence of what it is to be beautiful,; the mind recognizes the Form Itself directly.  But be careful to note that the Form itself is not an idea in a person's "mind"; it is a real entity existing "own its own" independently from any human (or other) being's idea of it. (This is why it is misleading to translate Platos "eidos" as the English "idea"; "ideas" as we use that term in English are clearly intended to be "mental," things in the mind.  Plato's "eidos" is most definitely not "in" any finite, changeable human mind. The word "Form" is a much preferable translation.)

As the object of knowledge the Form is "seen" or "recognized" with the "mind's eye" (this is how Plato refers to the faculty of nous) in analogy to the way in which opinions take as their objects the beautiful physical objects "seen" with the body's eyes.

Thus Forms are the objects of knowledge while physical objects are objects of opinions. The former are "seen" (in a metaphorical sense) with the mind, the latter with the senses. But it is essential to Plato's view that Forms are not themselves "in the mind" because minds (and their ideas) can change.  The Form as the object of knowledge cannot change; it must be forever, eternally, what-it-is-to-be that of which it is the Form.