In Plato's Republic, Socrates confidently asserts to Glaucon, Plato’s older brother, that evil cannot be done consciously, or rationally, for one doing evil believes himself to be doing good, and one cannot do evil to another, because one can only do good or evil to oneself:
The Reading Room
Evil In Plato’s Republic and Dante’s Paradiso
Socrates: The true quality of God we must always surely attribute to him whether we compose in epic, melic, or tragic verse.Glaucon: We must.Socrates: And is not God of course good in reality and always to be spoken of as such?
Glaucon: Certainly.
Socrates: But further, no good thing is harmful, is it?
Glaucon: I think not.
Socrates: Can what is not harmful harm?
Glaucon: By no means.
Socrates: Can that which does not harm do any evil?
Glaucon: Not that either.
Socrates: But that which does no evil would not be cause of any evil either?
Glaucon: How could it?
Socrates: Once more, is the good beneficent?
Glaucon: Yes.
Socrates: It is the cause, then, of welfare?
Glaucon: Yes.
Socrates: Then the good is not the cause of all things, but of things that are well it is the cause--of things that are ill it is blameless.
Glaucon: Entirely so, he said.
Socrates: Neither, then, could God, said I, since he is good, be, as the multitude say, the cause of all things, but for mankind he is the cause of few things, but of many things not the cause. For good things are far fewer with us than evil, and for the good we must assume no other cause than God, but the cause of evil we must look for in other things and not in God."(Plato, Republic II 379a-c)
One really would think that Dante read Plato's Republic, though Plato’s work exerted a larger influence on the medieval Arabic speaking world (Averroes’ Commentary on Plato’s Republic, for example) than in the medieval Latin-speaking world. But apparently truth needs no outside source, because, in Dante's Paradiso the very same, or at conspicuously similar, concept is proved in a deeply similar way. For one, for Dante suggests that there is an interlocking, and perhaps even parity between human nature and God’s nature by his representation of the trinity as having an inscription of man inside it, though he does not entirely demonstrate the point.
That circle—which, begotten so, appearedin You as light reflected—when my eyes
had watched it with attention for some time,
within itself and colored like itself,
to me seemed painted with our effigy,
so that my sight was set on it completely.
As the geometer intently seeks
to square the circle, but he cannot reach,
through thought on thought, the principle he needs,
so I searched that strange sight: I wished to see
the way in which our human effigysuited the circle and found place in it—
and my own wings were far too weak for that.
But then my mind was struck by light that flashed
and, with this light, received what it had asked.
(Par.33.127-141; my emphasis)
Second, Dante then explores the concepts of contingent will and absolute will.
At that point—I would have you see—the forcesto which one yielded mingles with one’s will;
and no excuse can pardon their joint act.
Absolute will does not concur in wrong;
but the contingent will, through fear that its
resistance might bring greater harm, consents.
Therefore, Piccarda means the absolute
will when she speaks, and I the relative;
so that the two of us have spoken truth.”
(Par.4.106-114)
One's contingent will is the will one uses every day to make the choices one makes. One might call it a "human will". The absolute will is the call or thrust of one's innermost nature which one shares with God and which never errs. Therefore, the absolute will is the manifestation within one of one's connection or identity with the Divine Nature, which is by Dante’s medieval definition perfect, and therefore always wills what is correct eternally. One might consider it similar to an unerring instinct. The goal, then, is for a human to align his contingent (or fallible) will with the absolute will in order to place his human nature in harmony with his divine nature. What does this mean? This means that each human's nature is perfect, and therefore no human’s nature is imperfect. Each human, however, is not simply nature or form, but a composition of matter and form for medieval scholastic thinkers. The mixture of matter with form leads to one's temporal and imperfect existence, and the existence of a contingent will, which allows for free choice and error, the greatest gift of God.
There are some interesting implications to these observations by Dante. If one's nature as a human is shared with god, then it must be perfect and therefore not imperfect or evil. One cannot be evil by nature. What, then, would make one evil? If one's contingent will turns from or acts against the absolute will, then one is doing evil or harm to one's own being, because one is straining the relationship between contingent will and absolute. For example, when Cain kills his brother Abel early in Genesis, Cain is doing evil to the absolute will by acting against it with his contingent will by killing his brother. How is this Platonic, though? When Cain kills his brother, to whom or what is Cain committing evil? The answer, surprisingly, is that Cain is not doing evil to his brother, but to himself. How can this be? If doing evil involves using one's contingent will against the absolute will, one can only ever do harm or evil to one's own being or self by choosing a path worse than that laid out by God, or the absolute will. Naturally, one can physically harm another being or abuse it in some way, but if doing evil is defined as one's will acting against the divine will, or absolute will, the only person one can ever do evil to is one's self by acting against one's truest nature, the nature one shares with God.