The privation of good is a theological doctrine that evil, unlike good, is insubstantial, so that thinking of it as an entity is misleading. Instead, evil is rather the absence or lack ("privation") of good. It is typically attributed to St. Augustine of Hippo, who wrote:
Our perceptions are based on contrast, so that light and dark, good and evil, are imperceptible without each other; in this context, these sets of opposites show a certain symmetry. But a basic study of optics teaches us that light has a physical presence of its own, whereas darkness does not: no "anti-lamp" or "flashdark" can be constructed which casts a beam of darkness onto a surface that is otherwise well-lit. Instead, darkness only appears when sources of light are extinguished or obscured, and only persists when an object absorbs a disproportionate amount of the light that strikes it.