Platonic idealism is a common philosophical concept that is readily embraced by a large portion of the population but is in reality an idea that makes psychological fulfillment nearly impossible.

Platonic idealism attempts to define concepts by some abstract perfect notion of their essence that all material manifestations of objects called by those names can only strive to meet. 

Consider something as simple as defining a rock.  You could start by saying a rock is hard, but not all rocks are, like gypsum. Or that rocks are heavy, but some are not, such as aerated pumice. The more generic one’s definitions are, the more things it includes that are not intended to be there, and the more specific definitions are, the more things are excluded and the less useful the definitions become.

Plato would argue that there is some abstract ideal ‘rockness’ that all real world rocks are but an imperfect manifestation of and the exact qualities of the perfect ‘rockness’ are unknowable to humans but if one navel gazes long enough they might catch a fleeting feeling of perfect rockness.

In other words, concepts are defined by something separate from reality AND separate from one’s imagination, but a "Third Order" (just as structuralism in sociology - apparently) 

Being separate from reality AND imagination means that there is some hyper reality that exists outside the realm of existence and imagination. But if real, then such a hyper-reality must itself be part of reality. Plato had the sense enough to argue Platonic Forms (the actual manifestations of the ideal) do in fact exist - but only in the realm of the Gods.

Platonic Idealism is found in many places, from defining rocks to classifying races and even to defining words. Today the science of Taxonomy wrestles deeply with platonic idealism in trying to clearly classify and identify species. Again the more specific one gets, the less organisms apply to it, while the more vague, the less useful the actual classification system is (see wikipedia’s "The Species Problem" http://en.wikipedia.org/wiki/Species_problem)

But as individuals we also wrestle with Platonic Idealism. If you’ve ever caught yourself pondering endlessly which meal to get when you go out to eat, striving for the ‘perfect’ one, you are striving for the Platonic Ideal of a meal, but you can’t really know for sure, because that ‘ideal’ meal is just outside the reach of your cognitive ability, but if you try hard enough you might just get one extra bit of information or catch a fleeting glimpse of a notion of which is perfect. A writer may agonize endlessly over that perfect sentence, editing and re-editing over and over.  A musician might spend days striving to make that really good composition absolutely *perfect* What is lost in these struggles is the notion that the perfect is not knowable and requires omniscience to even recognize and instead recognizing a ‘good enough’ or ‘best possible’ within the context of your knowledge and with a reasonable amount of time spent on the problem is much more productive and ultimately psychologically healthy.

Consider the protestation of “too much choice” that paternalistic do-gooders and psychologists now complain about. See the “Paradox of Choice” (http://tinyurl.com/4rfmnyg) where psychologists argue that ‘too much’ choice that capitalism has resulted in makes it too difficult to make informed decisions and results  in more unhappiness and lower self esteem because of the fear that they buyer made the wrong choice. But instead of recommending a more rational ‘good enough’ strategy to decision making that is proper to rational beings who exist in a real world and who are not omniscient, instead of challenging the notion that people ought to feel bad about not making that ‘perfect’ decision in the first place that would have required omniscience, they suggest that the actual number of products available should be limited by law…

In contrast to Platonic Idealism, rival ancient Greek philosopher Aristotle argued that concepts should be classified not on some abstract form of perfection, but instead of either the cause or purpose of the concept. A house then is not a structure with four walls made of wood with a slanted roof that people linger in, that could be an office or a temple, nor is a table something made of wood with four legs, it could have six and be made of stone…instead a house is something whose purpose is for people to live in made by a house builder with the purpose of being a house while a table is something made by a table builder for the purposes of doing table things.

In writing of the Four Causes which concepts ought to be evaluated by, Aristotle stood in stark contrast to the idealized Forms of Plato’s and gave us a much more useful tool for identifying and understanding the real world as rational non-sentient beings. Ironically the science of taxonomy which Aristotle basically founded is one suffering greatly from Platonic Idealism, and while Aristotle’s system is not perfect, major strides in rational categorization and understanding have come directly from the application of Aristotle’s Causes to defining the essence of concepts, and today most scientific definitions evolve toward one or a combination of Aristotle’s Causes. Thus a rock is not something that embodies the ideal rockness of hardness, heaviness, etc in varying levels of imperfection, but instead rocks are solid aggregates (material cause, i.e. what it is made of) of minerals (formal cause, i.e. the arrangement of the matter) that are formed through igneous, metamorphic, or sedimentary geological processes (their efficient cause, their primary source of change)