An idea is a form (such as a thought) formed by consciousness (including mind) through the process of ideation. Human capability to contemplate ideas is associated with the ability of reasoning, 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 or impulsiveness.

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 philosophy, there is no other term which has been used with so many different shades of meaning. The view that ideas exist in a realm separate or distinct from real life is referred to as innate ideas. Another view holds that we only discover ideas in the same way that we discover the real world, from personal experiences. The view that humans acquire all or almost all their behavioral traits from nurture (life experiences) is known as tabula rasa ("blank slate"). Most of the confusions in the way of ideas arise at least in part from the use of the term "idea" to cover both the representation percept and the object of conceptual thought. This can be illustrated in terms of the doctrines of innate ideas, "concrete ideas verses abstract ideas", as well as "simple ideas verses complex ideas".

Plato

main: Theory of Forms Plato was one of the earliest philosophers to provide a detailed discussion of ideas. He considered the concept of idea in the realm of metaphysics and its implications for 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

Descartes often wrote of the meaning of idea as an image or representation, often but not necessarily "in the mind", which was well known in the vernacular. In spite of the fact that Descartes is usually credited with the invention of the non-Platonic use of the term, we find him at first following this vernacular use.^b In his Meditations on First Philosophy he says, "Some of my thoughts are like images of things, and it is to these alone that the name 'idea' properly belongs." He sometimes maintained that ideas were innate 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. 1 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)2. 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."3