The following article is taken from St. Thomas Aquinas' Compendium of Theology. I will be focussing on his section under the heading 'Faith', 115-122.
This Theological Short is more properly philosophical as it follows a process of human reasoning alone which is open to each man whether he knows of, or accepts Christian revelation. As I taught in the previous Theological Short, certain truths available to the enquiring human mind are also revealed to us in order to safeguard us from error. Some of the truths that I will lay out here can be obtained from revelation as they consider the inherent goodness of creation which has been communicated to us from above.
First of all it is necessary for us to realise that we do not consider evil to be a nature, a reality in its own right. We do not acknowledge a principle of evil or of darkness which exists co-eternally with
goodness, such as the Gnostics taught. We reject the possibility of a Supreme Evil existing as a counterpart to the Supreme Good. This will be shown in the following line of philosophical enquiry. The Angelic Doctor notes that a nature, a thing, a substance is either a potentiality or an actuality, what could be or what currently is. The former seeks to become perfect, that is, to become actual. All things by nature seek good properly understood. We here do not speak of, simply, moral good, but the perfection belonging to a thing as a particular thing. As something shares in actuality it shares in goodness (a perfection), that is, being considered under the aspect of desirability. To mix in a contrary, in this case, 'evil' does not lead a thing to a new perfection but rather it suffers a defect. To understand this point it is vital to realise the significance of the truth outlined above that evil is not a nature. What is produced by this unhappy mixture is not a new positive form (although it does receive a new form) but the devastation, the corruption, the corrosion of a perfection. As things seek their own good and perfection, wishing to remain secure in it, they also wish (by nature, not simply voluntarily) to avoid what destroys it. Existence itself, an actuality in the mode of being, is itself a good which 'all things desire, sharing in good perfects every nature''.
We define genus as a class of beings, it is 'what something is'. Further we define species as 'what sort of thing it is'. Thomas asserts that the particular species is determined by a thing's form. Therefore, in the case of a moral agent (a man or an angel), the moral species, derived from their form, is obtained from the end sought out by their will.
In natural objects or beings, the corruption of one form is related to the reception of another form. The Angelic Doctor uses the example of fire and its effect on wood. What occurs when fire touches wood is that the wood itself suffers deformation, it is warped, ultimately it loses a perfection it previously possessed. The agency of fire, a good in itself, when in contact with wood causes the latter to lose its integrity and perfection.
As regards moral agents, we consider it to be an evil for them to seek out an end which is defective, one that is removed from a necessary perfection. Evil here derives from the privation rather than merely the object sought itself. We are told that good and evil are specific differences which distinguish a class of moral beings. 'What is perfect always belongs to the nature of good, and what is lesser always belongs to the nature of evil'.
We can not state entirely correctly that evil exists. What we truly mean is that we believe (created) good can be corrupted, that its perfection can be destroyed. It is itself in the nature of created things to be liable to such negative change, not because of an inherent defect in their creation but by the very fact of their being brought into existence out of nothing. This possibility arises from its nature's potentiality. They are contingent beings that require an agent in act to bring them into existence and to sustain them in it. This last mention of creatio ex nihilo is properly theological as it cannot be proven by human reason alone that we were created out of nothing.
A particularly good analogy of how evil is a corruption of a perfection of a good is shown by the example of blindness. The proper object of sight is colour, however due to blindness, the thing is incapable of activating fully this sense. Blindness can occur to various degrees, that is, to a deepening level of damage or diminishing of a perfection. The eye itself, although terribly impeded in its proper function, still remains good in its nature. 'Both form and potentiality for form are good, and a potentiality is the subject of privation, just as it is the subject of a form'. The subject in which it adheres is 'necessarily good, not that is is contrary to evil, but that it is a potentiality regarding evil'.
We must underline here that not every good, as I have implicitly stated above, can be susceptible to evil. Only those beings which may lose a perfection can. For God we must not admit the presence of evil or even the potentiality of evil as He is said to be pure act, incapable of change as this would imply imperfection, whether this be the capacity to develop perfection or the ability to be deprived of it.
Thomas goes on to remark that an evil as a privation can not be desired in itself (only per accidens) but only inasmuch as it connected to a certain good. However we may consider this point more fully in a later article.
Evil must be found in a particular good, it can not exist on its own. It is to be found in good as in its subject. Privation can only be found in a being and as such it must inhere in a good. We say that evil is found in something in the same sense as we say blindness is found in an eye, or more precisely 'in the subject of the power of sight'.
It is now necessary to move on to distinguish the various senses of the term 'evil' that we have been using. Usually in common speak we would define it as some outrageous act such as murder or rape, but we should not be limited to particular instances of moral evil. The key point in this article is that evil is a defect or a privation, a reduction in a subject's perfection or goodness. We may mean this in two ways, 1) its nature or 2) its operation, such as its movement towards its desired end. In the first case we can mean the aforementioned example of blindness and for the second we can speak of limping as an impediment in the operation of walking, an action which suffers from a defect. As regards moral evil, this can only be produced by a conscious and active moral agent who knows what it is doing. Its voluntary nature is essential. A forced act which is imposed from outwith such as rape does not constitute fornication or adultery. Thomas also notes that an action done out of ignorance does not count as a voluntary act as the agent is not aware of the things of which the said action consists. If a defect is done consciously by an active moral agent we speak of sin and the agent deserves punishment.
Finally we consider evil that has the nature of punishment. The removal of a good can be imposed on a moral agent against their will in order to serve as a remedy for sin and to alter the deficient will for disordered actions. This is in itself actually a good as it hopefully brings the will to a state of justice by making punishment more unpleasant than the desire for illicit action.