IDEA is ‘invariant form’. This word may also designate the philosophical ‘form’ in a more general way.
In Ancient Greece, Platon (Plato) starts a wide analysis of ideas (forms) and of the thinking process itself. In fact, in the dialogues Phaedo, Symposium, Republic, and Timaeus he suggests that there is an array of ideas or forms. Platon’s words to indicate such forms are <eidos> and <idea>. But we must be careful, because the latter might suggest the English term and notion of ‘idea’ (invention). And this interpretation would be misleading. According to Platon, forms are not mental entities, nor they are mind-dependent. They are entities whose existence and nature are understandable only by the ‘mind’. However, they do not depend on being so grasped in order to exist. In other words, an <idea> exists independently of anyone who has thoughts on this very <idea>. Moreover <idea> is what differentiate mere opinions from knowledge.
<Idea> is ‘invariant form’. Platon, the founder of the Academy, thinks that there are at least two worlds. Firstly, the apparent world of concrete objects, held by the senses, which constantly changes. Secondly, an unchanging and unseen world of forms or abstract objects, grasped by pure reason, which ground what is apparent. Material entities are transient and liable to contrary properties, while ideas are unchanging and stable. In many a passage Platon is close to assert that material things can only be the objects of opinion (<doxa>). Therefore, real knowledge only regards unchanging ideas. Furthermore, ideas for Platon seem to serve as ‘universals’.
Thus Platon’s forms represent types of things, as well as qualities, patterns and relations (objects). Just as singular stones, beds, trees, boxes, pencils and cars refer to objects in this world, ‘stoneness’, ‘bedness’, ‘treeness’ ‘boxeness’, ‘pencilness’ and ‘carness’ refer to objects in another world.