Platone, developing his thoughts wondered what was the proper object of science, concluding that they cannot be anything else than the ideas, Hence the theory of ideas. For Plato the idea is unchanging and perfect entity, It is a grey area with other ideas differ from ours (the supercelestial).
Things are, According to Plato, only copies of ideas.
The opinion is changeable and imperfect, reflects the things which are also changing and imperfect.
Science instead proves to be immutable and perfect, reflect unchanging and perfect ideas.
There are two basic types of ideas:
Ideas-values: match the Supreme ethical principles
Mathematical ideas: corresponding to the amount of arithmetic and geometry.
Pyramidal organization ideas Plato puts ideas values at the top of the pyramid and idea of good at the top.
The relationship between ideas and things:
Ideas are judging criteria of things (conceivability of condition objects) but also because of the things (condition of the existence of objects).
Between ideas and things there are well defined reports : mimesis (things imitate ideas), the eyesuponitaly (things involved in the essence of ideas) and the Parousia (ideas are present in things).
The ideas have to be the subject of intellectual vision, but where does this vision?
Plato uses the doctrine of reminiscence (meaning of remembrance): the soul before delving into our body is experienced in the world of ideas, where between one life and another might examine several specimens of things.
Arriving in our world it retains a memory damped, attenuated, of what he saw in the upper world, but only thanks to experience things she recalls what he saw in Europe.