Initially find to be puzzling, or even an egregious affront to common sense. Plato assumes, following Parmenides, that what is real may be thought and what is thought may be said. In other words, reality may be known through rational inquiry or thinking and the resultant thoughts may be communicated propositionally. But how do linguistically expressed judgments convey truths about non-linguistic realities?
Forms as class concepts.
If A is a certain woman and B is a certain statue and both “A is beautiful” and “B is beautiful” are true statements, one might be tempted to think of the woman and the statue as participating in or sharing some common property–beauty, despite their being otherwise quite different. This is the way Plato thinks, and he calls the common property ‘beauty itself,’ as distinct from the particular beauty of either woman or statue (unfortunately, translators often feel compelled to turn Plato’s ‘beauty itself’ into “ideal beauty” or “absolute beauty”). How can we intelligently say that some particular objects are beautiful unless we have a prior acquaintance with beauty itself, so that we can identify it in those objects? Knowledge of ‘the beautiful itself’ is a prerequisite for knowing whether ‘A is beautiful’ or ‘B is beautiful’ are true statements.
Philosophers talk of conceptual or logical priority as distinct from temporal priority, and this is the sort of firstness at issue here. If an understanding of x is necessary for an understanding of y, then x is conceptually or logically prior to y. It would seem that, if we are to understand what we are saying when we say ‘This woman is beautiful,’ we must understand what ‘beautiful’ means. Nor can we know whether that or any other statement is true (i.e., is an item of knowledge) unless we understood what the statement means. What then is the status of all those statements, constituting the vast majority of our assertions, which we make before we have established a clear understanding of the terms the statements contain? Plato would say that they must only be opinions, since they clearly cannot be instances of knowledge.
Forms as standards.
Neither the woman nor the statue of our example, nor any other concrete sensible object, is perfectly beautiful, for each of these objects is many things (for example, woman, red-headed, slim, graceful, with a wart on her left hand, etc.) and not just this one thing–beautiful. Only “the beautiful itself” is just beauty uncompounded with any other properties. The beautiful woman “participates in” or “shares” this beauty with all other beautiful things, but both she and all those other things can only be beautiful in certain respects and to a certain degree. The concept of beauty, or what Plato calls “the beautiful itself” or “Beauty,” provides a standard with which to judge individual objects as being more or less beautiful. Because they are the patterns or ideal models to which we compare individual things or actions in order to determine how beautiful, just, or whatever, they are, he also refers to them as ‘Forms’ or ‘Ideas.’ For this reason, Plato’s view has been called idealism.
Our knowledge of Forms.
It is clear that we cannot apprehend Forms by our senses. We see the beautiful person, but beauty itself is not something we can see or hear. We would say that we ‘have an idea’ or ‘have a concept’ of beauty; Plato writes of apprehending Forms with ‘the mind’ or ‘the eye of the soul’ rather than with the senses. We do hear beautiful pieces of music and see beautiful objects, but Plato’s point is that we are able to do so only because we have some idea of what beauty itself is. Even if hearing a sound is entirely an affair of the sense organs, hearing that sound as beautiful is to mentally classify it as having satisfied those ideal conditions, which would be specified in a definition of “beauty.” Some would say that our concepts are constructed by the mind by a process of selective abstraction from sensible experience–that, in effect, we make our ideas. This view has sometimes been called nominalism, because concepts are nothing more than names by which we conventionally designate certain sensible properties or patterns of sensible properties. Plato does not agree with this view. He thinks that our ideas are not like artefacts, but rather like perceptions. That is, just as visual perceptions are of objects, which exist outside us, our concepts are mental perceptions of non-sensible realities which likewise exist independently of us. This view has sometimes been called realism, because it takes mental objects to be objectively real.
Thinking this way, it makes perfectly good sense to say that ‘Beauty really exists’ and that it will be whatever it is regardless of how you or I may think of it. Plato thinks that if Beauty and Justice were only names and not realities then all our aesthetic and moral judgments would only express conventional prejudices and that none of them could be true. How would this consequence follow? Because, as Parmenides had argued, knowledge is apprehension of what really is. If ‘beauty’ or ‘justice’ are not realities, then statements such as “Symmetry is beautiful” and “Paying debts is just” couldn’t be evaluated as either true or false, because there would be no non-arbitrary, natural, standard, meaning of ‘beautiful’ or ‘just.’ Further, there is an obvious difference between perception and knowledge. Knowledge is mental or conceptual, not sensible, experience. If knowledge is the correct apprehension of what truly exits, and if Forms did not exist, there would not be anything to know, for the only existing objects would be sensible, rather than conceivable, realities.
If Plato is right, we are not entitled to think of reality in the conventional commonsense way, that is, to assume that that which is sensible is most real. That which really exists is to be apprehended only through thinking–by constructing and testing theories. Sensible objects could not possibly be real; they could at best be “copies” or “images” (as Plato calls them) of underlying realities which can be thought about but which cannot be perceived. In short, what we usually call “the real world” is not that at all, but is rather just a world of appearance or seeming. Only the Forms really exist, for they are the “causes” (in the sense of archetypal standards) of whatever intelligible properties are discernible in those sensible things, which seem to be most real. If we don’t know what beauty, or equality, or justice is ideally, how can we recognize particular instances of these?
Interestingly, this means that the examples we began by considering–statements such as “This woman is beautiful”–cannot ever be cases of knowledge, because the subject expression designates a sensible, rather than an intelligible, object. We could never be certain of more than that “This woman seems beautiful,” because this opinion relies on ever changing and always incomplete observational evidence. The only statements which could express genuine knowledge would be those whose subject terms, as well as their predicate terms, designated Forms. In logical jargon, knowledge can be expressed only in universal propositions, not in singular propositions (propositions whose subject refers to some particular thing rather than to a Form). Scientific statements, as well as the definitions of virtues sought by Socrates in Plato’s dialogues, are not about particular facts or objects but about universals.