Why does God allow evil? If God is eternal and omnipotent why evil exists? In philosophy, the existence of evil supposes for some authors the denial of God. To understand the problem of evil frst we must understand the metaphysics of Aristotle. Aristotle explains the difference between the act (the existence) and the potency(the capacity to do something). Aristoteles argues that the act of the good is better than the potency of the good, because in act we are sick or healthy but potentially we can be either healthy or sick, in the act of the good it cannot be simultaneously the opposites, they mutually excluded. This means that the act is more perfect and complete than the potency. On the other hand, in the evil the act is worse than the potency, in other words, there is a loss in the process from the later to the previous, at some point of the process we arrive at a good.
From this follows a conclusion that is very important for the Theology of Thomas Aquinas: "Evil does not exist separate from the sensible objects. "Consequently, between the primordial and eternal things, there is neither evil nor error nor corruption". Book IX, Chapter IX, Book of Metaphysics of Aristotle. Thomas Aquinas explains that evil needs a good as a support or base because every being is good, good is a nature because it is the final cause of the beings: the fruit is good when it is mature, the house It's good when it's finished. As for the evil Thomas Aquinas says: "we say bad to the man deprived of virtue." That is, for Metaphysics there is no "ontological evil" and this idea is retaken by Thomas Aquinas and St. Augustine in the scholastic philosophy.
San Agustin in his book Confessions analyzes what is the time and why evil exists. As for evil, he concludes saying that it exists simply because there is free will, because we have freedom of choice, from there the affirmation in the Bible that says:
God made man from the beginning, and left him in the hand of his own counsel.
Ecclesiasticus (The book of Sirach) 15:14