Plato’s Theory of Ideas: Intelligible and Sensible
Plato’s Theory of Ideas
The Theory of Ideas is at the heart of Platonic philosophy. Although it is not expressed as such in any single work, it is addressed from different perspectives in several of his later works, such as The Republic, Phaedo, and Phaedrus. This theory has at least three main intentions:
- Ethical Intention: Following Socrates, Plato seeks to ground virtue in knowledge. To be just, one must know what justice is. Against the moral relativism of the Sophists, Plato posited the existence of an eternal and immutable idea of justice, and of every other virtue.
- Political Intention: Rulers should be philosophers who are guided not by personal ambition, but by transcendent and absolute ideals.
- Scientific Intention: Plato acknowledges, with Protagoras, that sensory knowledge is relative. However, unlike Protagoras, who believed that sensory knowledge is the only form of knowledge, Plato, following Parmenides, believes that there is another form of knowledge, proper to reason, which is directed to a different object than that provided by the senses: the Ideas.
Nature of True Knowledge
True knowledge must be about Being, not Becoming, and it cannot be subject to error; it must be infallible. Science, therefore, can only be about stable and permanent objects. Sensory knowledge cannot be true knowledge because it does not meet these criteria.
Intelligible and Sensible Reality
Plato distinguishes two modes of reality:
- Intelligible Reality: This realm is eternal, indestructible, immaterial, unchanging, and serves as the model for the other reality. It is composed of Ideas, which represent the true essence of things.
- Sensible Reality: This realm consists of what we call “things.” It is corruptible, subject to change, generation, and corruption, and is merely a copy of intelligible reality.
This theory implies an ontological doubling. The sensible world is not a mere illusion; although its degree of reality is less than that of the Ideas, it cannot be equated to nothing.
Ascent to the Upper World
The ascent to the upper world, symbolized by the prisoner’s journey in Plato’s Allegory of the Cave, and the subsequent adaptation to the light to see the sun directly, represent the difficulties of the educational process. Dialectic and love are two paths to ascend to the world of Ideas. Mathematics is a necessary prelude, as it draws the soul away from the world of becoming and introduces it to the contemplation of intelligible objects. Within the myth, this corresponds to the ascent out of the cave, as it allows the transition from the sensible to the intelligible world.
The Role of Beauty
The soul, upon beholding the beauty of this world, remembers the truth and desires to take flight and see the Ideas. First, it is attracted to the beauty of a single body. Then, it realizes that beauty is the same in all bodies and moves to love beauty in general. Above this, it discovers the beauty of the soul, and higher still, the beauty of laws, then the beauty of sciences, and above all, Beauty itself, which is eternal, perfect, unchanging, and the source of all beauty.
Hierarchy of Ideas
In the myth, the prisoner discovers that the sun produces the seasons and years, governs the visible realm, and is, in a sense, the cause of all the things they saw. Ideas are ranked. The highest rank corresponds to the Idea of the Good, as presented in The Republic. In other dialogues, this place is taken by the One or Beauty, representing the maximum degree of reality and the cause of all that exists. Following these are the ideas of ethical and aesthetic objects, then the ideas of mathematical objects, and finally, the ideas of things.