Falar da teoria das ideias, e das formas, de Platão não é fácil, mas podemos apresentar aqui um resumo interessante. Os textos deste autor são, demasiadas vezes, tão crípticos que se torna difícil compreender as muitas teorias que o autor, através da voz de Sócrates, tenta transmitir, e disso são um perfeito exemplo a teoria das ideias e das formas. Porém, nas suas Epístolas Morais a Lucílio, Séneca o Jovem dá uma pequena ajuda, que aqui cito em tradução inglesa:
This "idea", or rather, Plato's conception of it, is as follows: "The 'idea' is the everlasting pattern of those things which are created by nature." I shall explain this definition, in order to set the subject before you in a clearer light: Suppose that I wish to make a likeness of you; I possess in your own person the pattern of this picture, wherefrom my mind receives a certain outline, which it is to embody in its own handiwork. That outward appearance, then, which gives me instruction and guidance, this pattern for me to imitate, is the "idea." Such patterns, therefore, nature possesses in infinite number - of men, fish, trees, according to whose model everything that nature has to create is worked out.
If you would [like to] know what "form" means, you must pay close attention, calling Plato, and not me, to account for the difficulty of the subject. However, we cannot make fine distinctions without encountering difficulties. A moment ago I made use of the artist as an illustration. When the artist desired to reproduce Vergil in colours he would gaze upon Vergil himself. The "idea" was Vergil's outward appearance, and this was the pattern of the intended work. That which the artist draws from this "idea" and has embodied in his own work, is the "form." Do you ask me where the difference lies? The former is the pattern; while the latter is the shape taken from the pattern and embodied in the work. Our artist follows the one, but the other he creates. A statue has a certain external appearance; this external appearance of the statue is the "form." And the pattern itself has a certain external appearance, by gazing upon which the sculptor has fashioned his statue; this is the "idea." If you desire a further distinction, I will say that the "form" is in the artist's work, the "idea" outside his work, and not only outside it, but prior to it.