To explain the truth of this matter is difficult indeed. Know that created things are of two kinds: material and spiritual, sensible and intelligible. That is, some are perceptible to the senses, while others are only perceived by the mind.
Sensible realities are those which are perceived by the five outer senses: So, for example, those outward things which the eye sees are called sensible. Intelligible realities are those which have no outward existence but are perceived by the mind. For example, the mind itself is an intelligible reality and has no outward existence. Likewise, all human virtues and attributes have an intelligible rather than a sensible existence; that is, they are realities that are perceived by the mind and not by the senses.
Briefly, intelligible realities such as the praiseworthy attributes and perfections of man are purely good and have a positive existence. Evil is simply their non-existence. So ignorance is the want of knowledge, error is the want of guidance, forgetfulness is the want of remembrance, foolishness is the want of understanding: All these are nothing in themselves and have no positive existence.
As for sensible realities, these are also purely good, and evil is merely their non-existence; that is, blindness is the want of sight, deafness is the want of hearing, poverty is the want of wealth, illness is the want of health, death is the want of life, and weakness is the want of strength.
Now, a doubt comes to mind: Scorpions and snakes are poisonous—is this good or evil, for they have a positive existence? Yes, it is true that scorpions and snakes are evil, but only in relation to us and not to themselves, for their venom is their weapon and their sting their means of defence. But as the constituent elements of their venom are incompatible with those of our bodies—that is, as these constituent elements are mutually opposed—the venom is evil, or rather, those elements are evil in relation to each other, while in their own reality they are both good.
To summarize, one thing may be evil in relation to another but not evil within the limits of its own being. It follows therefore that there is no evil in existence: Whatsoever God has created, He has created good. Evil consists merely in non-existence. For example, death is the absence of life: When man is no longer sustained by the power of life, he dies. Darkness is the absence of light: When light is no more, darkness reigns. Light is a positively existing thing, but darkness has no positive existence; it is merely its absence. Likewise, wealth is a positively existing thing but poverty is merely its absence.
It is thus evident that all evil is mere non-existence. Good has a positive existence; evil is merely its absence.