Why is there a world at all? Why should there be a plurality of beings? In the Timaeus, the Phaedo, and the Republic, the only answer that is given is that the Good is the cause of everything. The Form of the Good is the cause of all being. Everything is arranged the way it is because it is best that it should be this way. “That this is the supremely valid principle of becoming and of the order of the world, we shall most surely be right to accept from men of understanding,”47 and “…the good took over all that is visible.” 48
In Phaedo49 Socrates, confronted with the question: “Why is there becoming?” answers that it is, because sensible things “partake” of forms; the forms “become present” to them. When the form departs, the thing ceases to be what it was.50 According to Phaedo the meaning of the Anaxagorian formula “Mind ordered everything” is this: that the best or good is the cause of the order of things.
In the Republic (Book VI) the Form of Good is compared by Socrates to the sun, which moves everything. Finality, therefore, is all-pervasive in all becoming. In the entire universe there is no action, no movement, no striving, but for the sake of Good. The Form of Good, therefore, is the cause, the real cause of existence and change in all visible reality.
Toward the end of the sixth book of the Republic Socrates says: “In like manner the good may be said not only to be the author of knowledge of all things known, but of their being and essence, and yet the good is not essence, but far exceeds essence in dignity and power.”51
It is clear from the above statements that for Plato the good is the supreme cause of the becoming of each aspect of the universe. This universe can be understood only by tracing a benevolent design in it. The category of design belongs to a higher order of reality than the elements out of which the world is constructed. In the Phaedo we are told that the real reason why Socrates remains in prison and does not run away to Megara is not that his body, the bones and sinews, cannot be moved, but the belief that it is better to obey the laws of the country. His idea of moral Good makes him stay there.
The idea of Good is therefore the ultimate real cause of (1) the existence of the world; (2) the essence of the world as a totality, and also of each element in it; (3) the design of the world; (4) the design of each being in the world.
That is why we can say that for Plato the purposeful realization of the Good is the cause of all causes; it not only makes the cosmos what it is, but is also the cause of its very existence. In other words it explains why there is a world at all; not only why it is such, but why it exists at all.
This world is there because it “partakes of” the Idea of the Good. All elements of the universe are what they are, and are there because they are all organized, harmonized for the sake of the Good. But, that for the sake of which something is or acts is precisely the basis of purposiveness. So, we can say that in Plato the action of the Good is the causing of order, the ordering of chaotic elements so that they all cooperate towards Good. This, however, is the very kernel of the idea of purposiveness. In Plato, therefore, purposiveness is not only something acting externally, on an already fully constituted system of beings, but it truly causes their existence, their very being. As such, it is a constitutive principle of being.
Does the Platonic idea of finality imply the notion of creation? We know that the ancients did not have the concept of “creation ex nihilo” and Plato, himself, did not develop it either. At least, there is little in his writings to suggest he did. The idea of the Good is eternal, separate, above essence and existence. On the other hand, as already pointed out, Plato was convinced that every movement, every action, must ultimately come from a “psyche” pursuing some good.
The question occurs: How does the Idea of the Good, which we now can call the Final Cause of the cosmos, exercise its causality?
Plato uses phrases like: “is present to”, causes, or “the world partakes of the Idea.” But the notion is never fully elaborated. My suggestion is that, in Plato's mind, it can not be explained further. Not by mortals, anyway. On the other hand, since he was convinced that ultimately only a psyche can act purposively (and therefore all movement in the world must ultimately be originated by a psyche), he introduced the Demiurge in the Timaeus. The Idea of the Good can not be identified with God. That is why the Demiurge is necessary.
It seems that an analysis of the relationship between the Demiurge, the Ideas, and the phenomenal world will throw some light on this problem.