Whether ideas exist in the mind alone or as an extra-mental objective existence, whether ideas are generated or exist innately in the mind, whether some types of ideas (such as God, soul, and world: See Kant) should be considered special or basically the same, and other questions concerning ideas have been central issues in the history of philosophy. Questions regarding the nature, essence, origin, and types of ideas have been integrated and contextualized into each philosophical thought, both in ontology and epistemology, and the meaning of idea has thus been configured accordingly.
Plato asserted, for example, that ideas or forms ("eidos") are not simply images that exist in the mind, but they are permanent extra-mental forms with which Demiurge, the divine crafter, created the cosmos. Those ideas or forms are, according to Plato, also inscribed in the soul prior to experience. Medieval scholastics understood those ideas as the forms within God's mind by which the Creator created the universe. Modern philosophers since Descartes, however, interpreted ideas as mental images that exist within the mind of a cognitive subject. Ideas were often understood as representations of objects outside of mind. This concept of idea as a mental image is still held today.
The word "Idea" originates from the Greek, and it is the feminine form of, the word εἶδος (Greek eidos: something seen; form, shape; related to idein "to see," eidenai "to know" ). "Idea" meant at first a form, shape, or appearance and implied the "visual aspect" of things in classical Greek. Accordingly, ideas and forms are used interchangeably for Greek authors.
With Plato, idea and/or form became essential concepts in philosophy. The ontological status of idea or form, epistemological roles of ideas or forms, and their ethical implications became central issues in philosophy. In this article, Plato's concept and the modern understanding of ideas are introduced to illustrate two different approaches to ideas.
Plato concept of ideas or forms are often capitalized as "Ideas" or "Forms" to distinguish his distinct notion from the modern conception of ideas as mental images. In this section, the term Form is used. But Form and Idea both refer to the same Greek term "eidos." Plato's Theory of Forms asserts that Forms or Ideas, and not the material world of change known to us through sensation, possess the highest and most fundamental kind of reality. Plato spoke of Forms in formulating his solution to the problem of universals.
The English word "form" may be used to translate two distinct concepts with which Plato was concerned—the outward "form" or appearance of something (Greek eidos and idea in their conventional, nontechnical senses, or other terms such as morphē), and "Form" in a new, technical sense, apparently invented by Plato (esp. eidos, idea). These are often distinguished by the use of uncapitalized "form" and capitalized "Form," respectively. In the following summary passage, the two concepts are related to each other:
Suppose a person were to make all kinds of figures (schēmata) of gold… —somebody points to one of them and asks what it is (ti pot'esti). By far the safest and truest answer is [to say] that it is gold; and not to call the triangle or any other figures which are formed in the gold "these" (tauta) as though they had existence (hōs onta)… And the same argument applies to the universal nature (phusis) which receives all bodies (sōmata)—that must always be called the same; for, while receiving all things, she never departs at all from her own nature, and never… assumes a form (morphē) like that of any of the things which enter into her; … But the forms which enter into and go out of her are the likenesses (mimēmata) of real existences (tōn ontōn aei) modelled after their patterns (tupōthenta) in a wonderful and inexplicable manner…
The forms that we see, according to Plato, are not real, but literally mimic the real Forms. In the Allegory of the cave expressed in Republic they are called the shadows of real things. That which the observer understands when he views the mimics are the archetypes of the many types and properties (that is, of universals) of things we see all around us. They are not located in the object, which as far as Plato is concerned, is mere smoke and mirrors situated in space (which also is real).
The Greek concept of form precedes the attested language and is represented by a number of words mainly having to do with vision: the sight or appearance of a thing. The main words, εἶδος (eidos) and ἰδέα (idea) come from the Indo-European root *weid-, "see." Both words are in the works of Homer, the earliest Greek literature.
These meanings remained the same over the centuries until the beginning of philosophy, when they became equivocal, acquiring additional specialized philosophic meanings. The pre-Socratic philosophers, starting with Thales, noted that appearances change quite a bit and began to inquire into the essential existence of things, leading some to conclude that things were made of substances, which comprise the actually existing thing being seen. They began to question the relationship between the appearance and the essential existence of things, between the substance and the form; thus, the theory of matter and form (today's hylomorphism) was born. Starting with at least Plato, and possibly germinal in some of the presocratics, the forms were considered "in" something else, which Plato called nature (phusis). The latter seemed as a "mother" (matter from mater) of substances.
For Plato, as well as in general speech, there is a form for every object or quality in reality: forms of dogs, human beings, mountains, colors, courage, love, and goodness. While the notion of form served to identify objects, Plato went further and inquired into the Form itself. He supposed that the object is essentially or "really" the Form and that phenomena are mere shadows that mimic the Form; that is, momentary portrayals of the Form under different circumstances. The problem of the universals - how can one thing in general be many things in particular - was solved by presuming that Form was a distinct singular thing but caused plural representations of itself in particular objects. Matter was considered particular in itself.
These Forms are the essences of various objects: they are that without which a thing would not be the kind of thing it is. For example, there are countless tables in the world but the Form of table-ness is at the core; it is the essence of all tables. Plato held that the world of Forms is separate from our own world (the world of substances) and also is the true basis of reality. Removed from matter, Forms are the most pure of all things. Furthermore, Plato believed that true knowledge/intelligence is the ability to grasp the world of Forms with one's mind.
A Form is aspatial (outside the world) and atemporal (outside time).  Forms are aspatial in that they have no spatial dimensions, and thus no orientation in space, nor do they even (like the point) have a location. They are non-physical, but they are not in the mind, and are extra-mental.
A Form is an objective "blueprint" of perfection. The Forms are perfect themselves because they are unchanging. For example, say we have a triangle drawn on a blackboard. A triangle is a polygon with 3 sides. The triangle as it is on the blackboard is far from perfect. However, it is only the intelligibility of the Form "triangle" that allows us to know the drawing on the chalkboard is a triangle, and the Form "triangle" is perfect and unchanging. It is exactly the same whenever anyone chooses to consider it; however, the time is that of the observer and not of the triangle.
The Forms exist in a rarefied sector of the universe. For everything on Earth there is a formal counterpart:
But the true earth is pure (katharan) and situated in the pure heaven (en katharōi ouranōi) … and it is the heaven which is commonly spoken by us as the ether (aithera) … for if any man could arrive at the extreme limit … he would acknowledge that this other world was the place of the true heaven (ho alethōs ouranos) and the true light (to alethinon phōs) and the true earth (hē hōs alēthōs gē).
In comparison to it our Earth is "spoilt and corroded as in the sea all things are corroded by the brine." There the colors are "brighter far and clearer than ours; there is a purple of wonderful lustre, also the radiance of gold and the white which is in the earth is whiter than any chalk or snow." Moreover the plants are better: "and in this far region everything that grows - trees and flowers and fruits - are in a like degree fairer than any here." Gems lie about like ordinary stones: "and there are hills, having stones … more transparent, and fairer in color than our highly-valued emeralds and sardonyxes …." And for the humans, "… they have no disease, and live much longer than we do, and have sight, and hearing and smell … in far greater perfection. They converse with the gods and see the sun, moon and stars as they truly are …." Indeed, for Plato, "god" is identical to the Form of the Good.
Plato's main evidence for the existence of Forms is intuitive only and is as follows.
To understand Plato's argument from human perception, it is helpful to use the example of the color blue. We call both the sky and blue jeans by the same color: blue. However, clearly a pair of jeans and the sky are not the same color; moreover, the wavelengths of light reflected by the sky at every location and all the millions of blue jeans in every state of fading constantly change, and yet we somehow have an idea of the basic form Blueness as it applies to them. Says Plato:
But if the very nature of knowledge changes, at the time when the change occurs there will be no knowledge, and, according to this view, there will be no one to know and nothing to be known: but if that which knows and that which is known exist ever, and the beautiful and the good and every other thing also exist, then I do not think that they can resemble a process of flux, as we were just now supposing.
No one has ever seen a perfect circle, nor a perfectly straight line, yet everyone knows what a circle and a straight line are. Plato utilizes the tool-maker's blueprint as evidence that Forms are real:
… when a man has discovered the instrument which is naturally adapted to each work, he must expess this natural form, and not others which he fancies, in the material ….
Given that perceived circles or lines are not exactly circular or straight, and yet idea of a perfect circle or line directs the manufacturer, then it follows that there must exist the idea or Form of a perfect circle or line.
Plato was well aware of the limitations of his theory, as he offered his own criticisms of it in his dialogue Parmenides, in which Socrates is portrayed as a young philosopher acting as junior counterfoil to aged Parmenides.
The dialogue does present a very real difficulty with the Theory of Forms, which was overcome later by Aristotle (but not without rejecting the independently existing world of Forms). It is debated whether Plato viewed these criticisms as conclusively disproving the Theory of Forms. It is worth noting that Aristotle was a student and then a junior colleague of Plato; it is entirely possible that the presentation of Parmenides "sets up" for Aristotle; that is, they agreed to disagree.
The difficulty lies in the conceptualization of the "participation" of an object in a form (or Form). The young Socrates conceives of his solution to the problem of the universals in another metaphor, which though wonderfully apt, remains to be elucidated:
But exactly how is a Form like the day in being everywhere at once? The solution calls for a distinct form, in which the particular instances that are not identical to the form participate; i.e., the form is shared like the day in many places. The concept of "participate," represented in Greek by more than one word, is as obscure in Greek as it is in English. Plato hypothesized that distinctness meant existence as an independent being, thus opening himself up to the famous Third Man Argument of Parmenides, which proves that forms cannot independently exist and be participated. and 
If universal and particulars - say man or greatness - all exist and are the same, then the Form is not one but is multiple. If they are only like each other then they contain a form that is the same and others that are different. Thus if the Form and a particular are alike then there must be another, or third, man or greatness by possession of which they are alike. An infinite regression must result (consequently the mathematicians often call the argument the Third Man Regression); that is, an endless series of third men. The ultimate participant, greatness, rendering the entire series great, is missing. Moreover, any Form is not unitary but is composed of infinite parts, none of which is the proper Form.
The young Socrates (some may say the young Plato) did not give up the Theory of Forms over the Third Man but took another tack, that the particulars do not exist as such. Whatever they are, they "mime" the Forms, appearing to be particulars. This is a clear dip into representationalism, that we cannot observe the objects as they are in themselves but only their representations. That view has the weakness that if only the mimes can be observed then the real Forms cannot be known at all and the observer can have no idea of what the representations are supposed to represent or that they are representations.
Plato's later answer would be that men already know the Forms because they were in the world of Forms before birth. The mimes only recall these Forms to memory. Unfortunately the hidden world can in no way be verified in this lifetime and its otherworldness can only be a matter of speculation (in those times before the knowledge of revelation and faith).
The topic of Aristotelian criticism of Plato's Theory of Forms is quite extensive and continues to expand, for many reasons. First, Aristotle did not just criticize Plato but Platonism typically without distinguishing individuals. Moreover, rather than quote Plato directly he chose to summarize him often in one-liners that are not comprehensible without considerable exegesis, and sometimes not then. As a historian of prior thought, Aristotle often uses the prior arguments as a foil to present his own ideas. Consequently, in presenting the Aristotelian criticisms it is necessary to distinguish what Aristotle wrote, what he meant, what Plato meant, the validity of Aristotle's understanding of Plato's thoughts, and the relationship between Plato's thought and Aristotle's concepts: a formidable task extending over centuries of scholarship. This article presents a few sample arguments addressed by a few sample scholars. Readers may pursue the topic more fully through the citations and bibliography.
In the summary passage quoted above Plato distinguishes between real and non-real "existing things," where the latter term is used of substance. The figures, which the artificer places in the gold, are not substance, but gold is. Aristotle, after stating that according to Plato all things studied by the sciences have Form, asserts that Plato considered only substance to have Form giving rise to the contradiction of Forms existing as the objects of the sciences but not existing as non-substance.
Despite Ross's objection that Aristotle is wrong in his assumption, that Plato considers many non-substances to be Forms, such as Sameness, Difference, Rest, Motion, the criticism remains and is major, for it seems that Plato did not know where to draw the line between Form and non-Form. As Cornford points out, things about which the young Socrates (and Plato) asserted "I have often been puzzled about these things" referring to Man, Fire and Water, appear as Forms in his later works, but others do not, such as Hair, Mud, Dirt, about which Socrates is made to assert: "it would be too absurd to suppose that they have a Form."
Another argument of Aristotle attacked by Ross is that Socrates posits a Form, Otherness, to account for the differences between Forms. Apparently Otherness is existing non-existence: the Not-tall, the Not-beautiful, etc., so that every particular object participates in a Form causing it not to be one essence; that is, a Form to exclude the essence but allow all others. According to Ross, however, Plato never made the leap from "A is not B" to "A is Not-B." Otherness only applies to its own particulars and not to the other Forms; for example, there is no Form, Non-Greek, only particulars of Otherness that suppress Greek.
However, this objection does not evade the question. Whether or not Socrates meant that the particulars of Otherness are Not-Greek, Not-tall, Not-beautiful, etc., such a particular still operates only on specific essences. If it were a general exclusiveness every Form would be excluded and nothing be anything in particular. If the exclusion excludes one essence then either Otherness is not unitary or multiple Othernesses exist, each one excluding one essence. It is something and it is not something; it allows and does not allow, which are contradictory properties of the one Form.
Though familiar with insight, Plato had postulated that we know Forms through remembrance. Aristotle successfully makes epistemological arguments against this view. In Plato the particulars do not really exist. Countering "... for that which is non-existent cannot be known" Aristotle points out that proof rests on prior knowledge of universals and that if we did not know what universals are we would have no idea of what we were trying to prove and could not be trying to prove it. Knowledge of the universal is given from even one particular; in fact, the inductive method of proof depends on it.
This epistemology sets up for the main attack on Platonism (though not named) in Metaphysics. In brief, universal and particulars imply each other; one is logically prior or posterior to the other. If they are to be regarded as distinct, then they cannot be universal and particulars; that is, there is no reason to understand the universal from the objects that are supposed to be particulars. It is not the case that if a universal A might be supposed to have particulars a1, a2, etc., A is missing or a1, a2, etc. are missing. A does not exist at all and a1, a2, etc. are unrelated objects.
The concept of ideas as images in mind in modern philosophy appeared within the context of the Representative Theory of Perception, a common framework of thought in modern philosophy.
The Representative Theory of Perception, also known as Indirect realism, "epistemological dualism," and "The veil of perception," is a philosophical concept. It states that we do not (and can not) perceive the external world directly; instead we know only our ideas or interpretations of objects in the world. Thus, a barrier or a veil of perception prevents first-hand knowledge of anything beyond it. The "veil" exists between the mind and the existing world.
The debate then occurs about where our ideas come from, and what this place is like. An indirect realist believes our ideas come from sense data of a real, material, external world. The doctrine states that in any act of perception, the immediate (direct) object of perception is only a sense-datum that represents an external object.
Aristotle was the first to provide an in-depth description of Indirect realism. In his work, On the Soul, he describes how the eye must be affected by changes in an intervening medium rather than by objects themselves. He then speculates on how these sense impressions can form our experience of seeing and reasons that an endless regress would occur unless the sense itself were self aware. He concludes by proposing that the mind is the things it thinks. He calls the images in the mind "ideas."
The way that indirect realism involves intermediate stages between objects and perceptions immediately raises a question: How well do sense-data represent external objects, properties, and events? Indirect realism creates deep epistemological problems, such as solipsism and the problem of the external world. Nonetheless, Indirect realism has been popular in the history of philosophy and has been developed by many philosophers including Bertrand Russell, Spinoza, René Descartes, and John Locke.
In striking contrast to Plato’s use of idea  is that of John Locke in his masterpiece Essay Concerning Human Understanding in the Introduction where he defines idea as "It being that term which, I think, serves best to stand for whatsoever is the object of the understanding when a man thinks, I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about in thinking ; and I could not avoid frequently using it." He said he regarded the book necessary to examine our own abilities and see what objects our understandings were, or were not, fitted to deal with. In his philosophy other outstanding figures followed in his footsteps - Hume and Kant in the eighteenth century, Arthur Schopenhauer in the nineteenth century, and Bertrand Russell, Ludwig Wittgenstein, and Karl Popper in the twentieth century. Locke always believed in good sense - not pushing things to extremes and taking fully into account the plain facts of the matter. He considered his common sense ideas "good-tempered, moderate, and down-to-earth." c
Hume differs from Locke by limiting "idea" to the more or less vague mental reconstructions of perceptions, the perceptual process being described as an "impression." Hume shared with Locke the basic empiricist premise that it is only from life experiences (whether our own or other's) that out knowledge of the existence of anything outside of ourselves can be ultimately derived. We shall carry on doing what we are prompted to do by our emotional drives of all kinds. In choosing the means to those ends we shall follow our accustomed association of ideas.d Hume is quoted as saying: "Reason is the slave of the passions."
The history of ideas is a field of research in history that deals with the expression, preservation, and change of human ideas over time. The history of ideas is a sister-discipline to, or a particular approach within, intellectual history. Work in the history of ideas may involve interdisciplinary research in the history of philosophy, the history of science, or the history of literature. In Sweden, the history of ideas has been a distinct university subject since the 1930s, when Johan Nordström, a scholar of literature, was appointed professor of the new discipline at Uppsala University. Today, several universities across the world provide courses in this field, usually as part of a graduate program.
The historian Arthur O. Lovejoy (1873–1962) coined the phrase history of ideas and initiated its systematic study, in the early decades of the twentieth century. For decades Lovejoy presided over the regular meetings of the History of Ideas Club at Johns Hopkins University, where he worked as a professor of history from 1910 to 1939.
Aside from his students and colleagues engaged in related projects (such as René Wellek and Leo Spitzer, with whom Lovejoy engaged in extended debates), scholars such as Isaiah Berlin, Michel Foucault, Christopher Hill, J. G. A. Pocock and others have continued to work in a spirit close to that with which Lovejoy pursued the history of ideas. The first chapter/lecture of Lovejoy's book The Great Chain of Being lays out a general overview of what is intended (or at least what he intended) to be the program and scope of the study of the history of ideas.
Lovejoy's history of ideas takes as its basic unit of analysis the unit-idea, or the individual concept. These unit-ideas work as the building-blocks of the history of ideas: though they are relatively unchanged in themselves over the course of time, unit-ideas recombine in new patterns and gain expression in new forms in different historical eras. As Lovejoy saw it, the historian of ideas had the task of identifying such unit-ideas and of describing their historical emergence and recession in new forms and combinations.
Quentin Skinner has been influential with his critique of Lovejoy's "unit-idea" methodology. Instead, he proposes a sensitivity to the cultural context of the texts being analysed and the ideas they contained.
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