Una '''idea''' és un producte del pensament, l'expressió d'un concepte.

==Les idees en filosofia==
Les idees en filosofia equivalen a l'essència del coneixement. El terme neix amb Plató i la seva divisió del món entre el sensible i el de les idees, l'autènticament real. L'idealisme és el corrent que defensa la seva existència per sobre de les aparences dels sentits. Segons els racionalistes, les idees són innates, són l'activitat mental pròpia de l'home, a diferència dels animals. En canvi per als empiristes són simplement noms, abstraccions per designar un conjunt de processos mentals a partir de sintetitzar dades de l'experiència, són sempre productes secundaris, etiquetes. 

==Les idees en psicologia==
Les idees són ocurrències, pensades o continguts de la ment. Suposa una resposta del cervell a un problema (té una idea per resoldre una situació) o una nova invenció. Les idees poden ser comunicades a altres persones i poden influir en la conducta sota la forma de prejudicis, motivació per aconseguir una fita o projecció del que pot passar. 

An '''idea''' is a form (such as a ideation. Human capability to contemplate ideas is associated with the ability of self-reflection, and of the ability to acquire and apply intellect, intuition, inspiration, etc.. Further, ideas give rise to actual concepts, or mind generalisations, which are the basis for any kind of knowledge whether science or philosophy.

In a popular sense, an idea arises in a reflex, spontaneous manner, even without thinking or serious reflection, for example, when we talk about the ''idea'' of a person or a place.

==Philosophy==
In separate or distinct from concrete ideas verses abstract ideas", as well as "simple ideas verses complex ideas". <ref>''The Encyclopedia of Philosophy'', MacMillian Publishing Company, New York, 1973 ISBN 0028949501 ISBN 978-0028949505 Vol 4: 120 - 121</ref>

===Plato===

idea in the realm of epistemology.  He asserted that there is realm of Forms or Ideas, which exist independently of anyone who may have thought of these ideas.  Material things are then imperfect and transient reflections or instantiations of the perfect and unchanging ideas.  From this it follows that these Ideas are the principal reality (see also idealism). In contrast to the individual objects of sense experience, which undergo constant change and flux, Plato held that ideas are perfect, eternal, and immutable.  Consequently, Plato considered that knowledge of material things is not really knowledge; real knowledge can only be had of unchanging ideas.

===René Descartes===

innate <ref>Vol 4: 196 - 198</ref> and uses of the term ''idea'' diverge from the original primary scholastic use. He provides multiple non-equivalent definitions of the term, uses it to refer to as many as six distinct kinds of entities, and divides ''ideas'' inconsistently into various genetic categories. [http://plato.stanford.edu/entries/descartes-ideas] For him knowledge took the form of ideas and philosophical investigation is the deep consideration of these ideas. Many times however his thoughts of knowledge and ideas were like those of Plotinus and Neoplatonism. In Neoplatonism the Intelligence (''Nous'') is the true first principle -- the determinate, referential 'foundation' (''arkhe'') -- of all existents; for it is not a self-sufficient entity like the One, but rather possesses the ability or capacity to contemplate both the One, as its prior, as well as its own thoughts, which Plotinus identifies with the Platonic Ideas or Forms (''eide'')[http://www.iep.utm.edu/p/plotinus.htm]. A non-philosophical definition of ''Nous'' is '''good sense''' (a.k.a. "common sense"). Descartes is quoted as saying, "Of all things, '''good sense''' is the most fairly distributed: everyone thinks he is so well supplied with it that even those who are the hardest to satisfy in every other respect never desire more of it than they already have."[http://en.wikiquote.org/wiki/Ren%C3%A9_Descartes]

===John Locke=== 
In striking contrast to Plato’s use of idea <ref>Vol 4: 487 - 503</ref> is that of John Locke in his masterpiece An 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 18th century, Arthur Schopenhauer in the 19th century, and Bertrand Russell, Ludwig Wittgenstein, and Karl Popper in the 20th century. Locke always believed in '''good sense''' - not pushing things to extremes and on taking fully into account the plain facts of the matter. He considered his common sense ideas "good-tempered, moderate, and down-to-earth."  <sup>c</sup>

===David Hume===
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."<ref>Vol 4: 74 - 90</ref> 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.<sup>d</sup> Hume is quoted as saying: "Reason is the slave of the passions."
left|thumb|135 px|"Modern Book Printing" from the [[Walk of Ideas]]

=== Immanuel Kant ===
autonomy of the rational and universal subject is opposed to the determinism of the empirical subject.<ref>Vol 4: 305 - 324</ref> Kant felt that it is precisely in knowing its limits that philosophy exists. The business of philosophy he thought was not to give rules, but to analyze the private judgements of good common sense.<sup>e</sup> 

===Rudolf Steiner===
Whereas Kant declares limits to knowledge ("we can never know the thing in itself"), in his epistemological work, Rudolf Steiner sees ''ideas'' as "objects of experience" which the mind apprehends, much as the eye apprehends light. In "Goethean Science" (1883), he declares, "Thinking… is no more and no less an organ of perception than the eye or ear. Just as the eye perceives colors and the ear sounds, so thinking perceives ideas." He holds this to be the premise upon which Goethe made his natural-scientific observations.

===Wilhelm Wundt===
assimilation and dissimilation (i.e. in color and form perception and his advocacy of ''objective'' methods of expression and of recording results, especially in language. Another is the principle of heterogony of ends - that multiply motivated acts lead to unintended side effects which in turn become motives for new actions.<ref>Vol 8: 349 -351</ref>

===Charles Sanders Peirce===
"How to Make Our Ideas Clear" (1878) and "The Fixation of Belief" (1877) <ref>Pierce's pragmatism</ref>. In "How to Make Our Ideas Clear" he proposed that a ''clear idea'' (in his study he uses concept and ''idea'' as synonymic) is defined as one, when it is apprehended such as it will be recognized wherever it is met, and no other will be mistaken for it. If it fails of this clearness, it is said to be obscure. He argued that to understand an idea clearly we should ask ourselves what difference its application would make to our evaluation of a proposed solution to the problem at hand. Pragmatism (a term he appropriated for use in this context), he defended, was a method for ascertaining the meaning of terms (as a theory of meaning). The originality of his ideas is in their rejection of what was accepted as a view and understanding of knowledge by scientists for some 250 years, i.e. that, he pointed, knowledge was an impersonal fact. Peirce contended that we acquire knowledge as ''participants'', not as ''spectators''. He felt "the real" is which, sooner or later, information acquired through ideas and knowledge with the application of logical reasoning would finally result in. He also published many papers on logic in relation to ''ideas''.

===G. F. Stout and J. M. Baldwin===
G. F. Stout and J. M. Baldwin, in the ''Dictionary of Philosophy and Psychology'' [http://psychclassics.yorku.ca/Baldwin/Dictionary], define "idea" as "the reproduction with a more or less adequate image, of an object not actually present to the senses." They point out that an idea and a perception are by various authorities contrasted in various ways. "Difference in degree of intensity", "comparative absence of bodily movement on the part of the subject", "comparative dependence on mental activity", are suggested by psychologists as characteristic of an idea as compared with a perception.

It should be observed that an idea, in the narrower and generally accepted sense of a mental reproduction, is frequently composite. That is, as in the example given above of the idea of chair, a great many objects, differing materially in detail, all call a single idea. When a man, for example, has obtained an idea of chairs in general by comparison with which he can say "This is a chair, that is a stool", he has what is known as an "abstract idea" distinct from the reproduction in his mind of any particular chair (see abstraction). Furthermore a complex idea may not have any corresponding physical object, though its particular constituent elements may severally be the reproductions of actual perceptions. Thus the idea of a centaur is a complex mental picture composed of the ideas of man and horse, that of a mermaid of a woman and a fish.
htpp://wedofree.com

== In anthropology and the social sciences ==
Diffusion studies explore the spread of ideas from culture to culture. Some anthropological theories hold that all cultures imitate ideas from one or a few original cultures, the Adam of the Bible or several cultural circles that overlap. Evolutionary diffusion theory holds that cultures are influenced by one another, but that similar ideas can be developed in isolation.

In mid-20th century, social scientists began to study how and why ideas spread from one person or culture to another. Everett Rogers pioneered diffusion of innovations studies, using research to prove factors in adoption and profiles of adopters of ideas. In 1976, Richard Dawkins suggested applying biological evolutionary theories to spread of ideas. He coined the term 'meme' to describe an abstract unit of selection, equivalent to the gene in evolutionary biology.

==Semantics==
===Dr. Samuel Johnson===
James Boswell recorded Dr.Samuel Johnson' s opinion about ideas. Johnson claimed that they are mental images or internal visual pictures. As such, they have no relation to words or the concepts which are designated by verbal names.

==Validity of ideas== 

In the objective worth of our ''ideas'' there remains the problem of the validity. As all cognition is by ''ideas'', it is obvious that the question of the validity of ''our ideas'' in this broad sense is that of the truth of our knowledge as a whole. Otherwise to dispute this is to take up the position of scepticism. This has often been pointed out as a means intellectual suicide. Any chain of reasoning (common sense) by which it is attempted to demonstrate the falsity of our ''ideas'' has to employ the very concept of ''ideas'' itself.  Then insofar as it demands assent to the conclusion, it implies belief in the validity of all the ''ideas'' employed in the premises of the argument. 

To assent the fundamental mathematical and logical axioms, including that of the principle of contradiction, implies admission of the truth of the ''ideas'' expressed in these principles. With respect to the objective worth of ideas, as involved in perception generally, the question raised is that of the existence of an independent material world comprising other human beings. The idealism of David Hume and John Stuart Mill would lead logically to solipsism (the denial of any others besides ourselves). The main foundation of all idealism and scepticism is the assumption (explicit or implicit), that the mind can never know what is outside of itself. This is to say that an ''idea'' as a cognition can never go outside of itself. This can be further expressed as we can never reach to and mentally apprehend anything outside of anything of what is actually a present state of our own consciousness.

* First, this is based on a prior assumption for which no real proof is or can be given 
* Second, it is not only not self-evident, but directly contrary to what our mind affirms to be our direct intellectual experience. 

What is possible for a human mind to apprehend cannot be laid down beforehand. It must be ascertained by careful observations and by study of the process of cognition. This postulates that the mind cannot apprehend or cognize any reality existing outside of itself and is not only a self-evident proposition, it is directly contrary to what such observation and the testimony of mankind affirms to be our actual intellectual experience. 

universal concepts, only the theory of moderate realism adopted by Saint Thomas can claim to guarantee objective value to our ideas. According to the nominalist and conceptualist theories there is no true correlate in ''rerum naturâ'' corresponding to the universal term. 

Mathematics, astronomy, physics, chemistry, and the rest claim that their universal propositions are true and deal with realities. It is involved in the very notion of science that the physical laws formulated by the mind do mirror the working of agents in the external universe. The general terms of these sciences and ''the ideas'' which they signify have objective correlatives in the common natures and essences of the objects with which these sciences deal.
Otherwise these general statements are unreal and each science is nothing more than a consistently arranged system of barren propositions deduced from empty arbitrary definitions. These postulates then have no more genuine objective value than any other coherently devised scheme of artificial symbols standing for imaginary beings. However the fruitfulness of science and the constant verifications of its predictions are incompatible with such a hypothesis.[http://www.newadvent.org/cathen/07630a.htm]

==Relationship of ideas to modern legal time- and scope-limited monopolies==