In Thomas Aquinas’s philosophical and theological system, good and evil are not equal and opposite forces. Instead, he understands evil as a privation, or a lack, of good. It’s not a thing or substance in itself, but rather a corruption or distortion of the good.
When Aquinas says, “Good can exist without evil, whereas evil cannot exist without good,” he is expressing this view. In his understanding, good is primary and fundamental. Goodness aligns with the nature of God, the ultimate good, and the intended order of creation. Everything that exists is good insofar as it exists, because existence itself is a good. Evil, on the other hand, isn’t a thing itself, but the absence or perversion of the good. It’s a departure from the fullness of being that things should have. It’s like a shadow that can’t exist without the light, or a hole that is defined by the substance around it.
So, while good can exist on its own, evil always relies on good for its existence. This idea differs from some other religious or philosophical systems, which might view good and evil as dualistic or equal opposing forces. For Aquinas, good has primacy, and evil is a secondary, parasitic reality.