7. But why does the existence of the Principle of Good necessarily comport the existence of a Principle of Evil? Is it because the All necessarily comports the existence of Matter? Yes: for necessarily this All is made up of contraries: it could not exist if Matter did not. The Nature of this Kosmos is, therefore, a blend; it is blended from the Intellectual-Principle and Necessity: what comes into it from God is good; evil is from the Ancient Kind which, we read, is the underlying Matter not yet brought to order by the Ideal-Form.
But, since the expression "this place" must be taken to mean the All, how explain the words "mortal nature"?
The answer is in the passage [in which the Father of Gods addresses the Divinities of the lower sphere], "Since you possess only a derivative being, you are not immortals... but by my power you shall escape dissolution."
The escape, we read, is not a matter of place, but of acquiring virtue, of disengaging the self from the body; this is the escape from Matter. Plato explains somewhere how a man frees himself and how he remains bound; and the phrase "to live among the gods" means to live among the Intelligible-Existents, for these are the Immortals.
There is another consideration establishing the necessary existence of Evil.
Given that The Good is not the only existent thing, it is inevitable that, by the outgoing from it or, if the phrase be preferred, the continuous down-going or away-going from it, there should be produced a Last, something after which nothing more can be produced: this will be Evil.
As necessarily as there is Something after the First, so necessarily there is a Last: this Last is Matter, the thing which has no residue of good in it: here is the necessity of Evil.