Evil (al-shar), concludes the Shaykh at Al-Shifa’; Ilahiyyat, IX.6.3.20-3,
essentially is nonexistence, though not every nonexistence but only the nonexistence of that to which the nature of the thing necessarily leads in terms of the perfections that belong permanently to its species and nature.
Existence is convertible with good. Evil, on the other hand, is not nonexistence tout court; rather, it’s the nonexistence, as he says, of a perfection which by nature belongs to some existent (mawjud). Now a perfection is (existentially) positive in nature; as such, it is good. Hence, evil is the nonexistence of some good that by nature belongs to some existent. This type of nonexistence is called privation. Evil, then, is privation.
From this, it follows that views that explain the ‘what’ of evil as either ontological separation, limitation, or negation are false. It isn’t mere limitation in existence and hence goodness; for there are (or, if you’re doubtful, there can be) things limited by nature – insofar as they aren’t God, who alone is Absolute Goodness – but which nevertheless actualize all of their perfections. That is, they don’t lack any of the limited perfections that they by nature ought to have. And it’s not the case that ‘absolute goodness’ is a perfection that a contingent existent by nature ought to have. Evil also isn’t ontological separation from God; for separation just denotes the fact of multiplicity in existence. But remember: existence and good are convertible; hence, the multiplicity, qua existing, is good. Nor, finally, is evil (logical) negation, i.e., of a thing being not-God. The fact that a rock is not an animal – and, more generally, one thing is not another – does not entail that that ‘not-being’ is an evil for the rock or the animal. But the fact that the animal is not seeing is an evil. Why? Because it is something that by nature ought to see. And as I’ve said, no creature by nature ought to be God (which means have His existence, goodness, etc.).
Evil, then, is the nonexistence of some good which an existent ought to have given its essence. This type of absence, i.e., of existence from that essence, is privation.