I. 1. The ontological dualism
The theory of the Ideas is the base of Plato’s philosophy: the Ideas are not only the real objects ontologically speaking, but they are the authentically objects of knowledge epistemologically speaking. From the point of view of ethics and politics, they are the foundation of the right behaviour, and anthropologically speaking they are the base of Plato’s dualism and they even allow him demonstrate the immortality of the soul. Plato defends a clear ontological dualism in which there are two types of realities or worlds: the sensible world and the intelligible world or, as he calls it, the world of the Ideas. The Sensible World is the world of individual realities, and so is multiple and constantly changing, is the world of generation and destruction; is the realm of the sensible, material, temporal and space things. On the contrary, the Intelligible World is the world of the universal, eternal and invisible realities called Ideas (or "Forms"), which are immutable and do not change because they are not material, temporal or space. Ideas can be understood and known; they are the authentic reality. The Ideas or Forms are not just concepts or psychic events of our minds; they do exist as objective and independent beings out of our consciences. They are also the origin of sensible things, but although they are the authentic beings, Plato, unlike Parmenides of Elea, do not completely deny the reality of the sensible things; the sensible world, although ontologically inferior, have also certain kind of being which comes from its participation or imitation of the world of Forms. The task of Demiurge is to give the shape of the Forms to that shapeless sensible material that has always existed making it thus similar to the Ideas. The Ideas are hierarchically ordered; there are different types and they do not have all the same value. The coherency of the arguments Plato uses for defending the existence of the Ideas would have lead him to claim there are Ideas of all those general words of which we can find an example in the sensible world, that is to say, of all the universal terms such as "justice", "rightness" or "man", but also terms as "table", "hair" or "mud". In spite of it, the population of Ideas postulated by Plato is limited enough by value considerations. Sorts of Ideas that are included in the intelligible world: the Idea of Rightness and other moral Ideas (Justice, Virtue, etc.); Aesthetic Ideas (specially the Idea of Beauty), Ideas of Multiplicity, Unity, Identity, Difference, Being, Not being, mathematical Ideas and other Ideas (the Idea of Man, etc.). Plato locates the Idea of Rightness on the highest position of that intelligible world; sometimes he identifies it with the Idea of Beauty and even with the idea of God. The Idea of Rightness is the origin of the existence of everything because human behaviour depends on it and everything tends to it (intrinsic purpose in the nature).
I. 2. Plato’s arguments in favour of the Theory of the Ideas In essence, this theory defends there are certain independent, universal, immutable and absolute beings which are different from the sensible world. a) Critic of the sensible knowledge in the dialogue "Theaetetus": Plato shows evidence does not rise from sensible knowledge. This kind of knowledge leads to relativism, which is, in essence, absurd (critic of sophist philosophy). Besides, we have knowledge not based on the senses. Conclusion: science (knowledge strictly talking) based on sensation as criterion for truth is not possible, because we cannot have science of changeable things (of the sensible world) which just appears to our senses. Science has to be based on reason, which studies the nature or essence of things ("Ideas").
b) The use of the language and the problem of the reference of the universal terms. Linguistic terms...