4. Every cause is either material, formal, efficient, or final. But evil can be neither matter nor form: for it has been shown (Chapp. VII, IX) that both actual being and potential being is good. In like manner evil cannot be an efficient cause, since everything acts according as it is actually and has a form. Nor can it be a final cause, since it is beside the intention (Chap. IV). Evil therefore cannot be the cause of anything; and therefore, if there be any cause of evil, it must be caused by good.
But since good and evil are opposites, and one opposite cannot be cause of another except accidentally, it follows that good cannot be the active cause of evil except accidentally. In physics, this accident may happen either on the part of the agent or on the part of the effect. On the part of the agent, when the agent suffers from a lack of power, whence it follows that the action is defective and the effect deficient. But to an agent, as such, it is quite an accident to suffer from a lack of power: for an agent does not act inasmuch as power is lacking to him, but according as he has anything of power. Thus then evil is caused accidentally on the part of the agent, inasmuch as the agent runs short of power: therefore it is said that evil has not got an efficient cause, but a deficient cause, because evil does not follow from an efficient cause except in so far as it is deficient in power, and in this respect is is not efficient. It comes to the same thing if the defect of the action and effect arises from some defect of the instrument, or of any other thing requisite for the agent's action, as when motive power produces halting through crookedness of the shin-bone: for the agent acts by both the power and the instrument. On the part of the effect evil is caused accidentally as well in respect of the matter of the effect as also in respect of its form. For if the matter is indisposed to receive the impression of the agent, some defect must follow in the effect. Nor is it imputable to any defect of the agent, that it does not transmute an indisposed matter to a perfect act: for the power of every natural agent is determined according to the limit of its nature; and its failure to transcend that cannot be brought in against it as a defect in power: such defect can then only be argued when it falls short of the measure of power due to it by nature. On the part of the form of the effect evil is accidentally incident, inasmuch as one form necessarily involves the privation of another form, and with the production of one thing there must needs ensue the destruction of another. But this evil does not belong to the effect intended by the agent, but attaches to something else. In the processes of nature therefore evil is caused by good only accidentally. The same is the case also in the processes of art: for art in its operation imitates nature, and is at fault in the same way as nature.
But in moral matters the case seems to be different. For a flaw in morals does not follow from any lack of power, seeing that weakness either entirely removes, or at least diminishes, moral reprehensibleness: for weakness does not deserve the punishment which is due to fault, but rather compassion and indulgence: to be blameworthy, a point of conduct must be a voluntary act, not an inevitable necessity. On careful consideration we find that the case of morals is in some respects like, in some respects unlike the case of physics. The unlikeness consists in this, that a moral fault is viewed as consisting in the action alone, not in any effect produced: for moral virtues are not effective, but active, while arts are effective; and therefore it has been said that art is at fault in the same way as nature. Moral evil therefore is not estimated according to the matter and form of the effect, but follows simply from the agent. Now in moral actions there are found in orderly enumeration four active principles. One principle is the executive power, namely, the motor power which moves the limbs to execute the command of the will. This power is moved by the will, and so the will is another principle. The will is moved by the judgement of the apprehensive faculty, which judges the particular thing proposed to be good or bad. -- good and bad being the (formal) objects of the will, the one object of seeking, the other of avoidance. Lastly, the apprehensive faculty is moved by the thing apprehended. The first active principle then in moral actions is the thing apprehended; the second is the apprehensive faculty; the third is the will; the fourth is motor power which executes the command of reason. But the act of the executive power already presupposes moral good or evil;* for these exterior acts bear a moral character only in so far as they are voluntary. Hence if the act of the will is good, the exterior act will also be called good; and evil, if the volition is evil. It would be no point of moral badness for the exterior act to fail by some defect unconnected with the will: thus lameness is not a moral but a natural blemish. Such a lack of executive power diminishes, if it does not totally excuse from, moral blemish. Again, the act whereby the object moves the apprehensive faculty is exempt from moral blemish: for it is according to the order of nature that what is visible affects the sight, and every object affects the corresponding passive potentiality. Even the act of the apprehensive faculty, considered in itself, is nowise morally blameworthy, as we see that any defect in it excuses from or diminishes moral blame, like the lack of executive power: for infirmity and ignorance alike excuse from sin, or diminish it. It remains then that moral blameworthiness is found first and foremost in the act of the will alone; and reasonably so, since an act is called 'moral' from being voluntary. In the act of the will then is to be sought the root and origin of what in the moral order is sin.
But this investigation leads us into an apparent difficulty. On the understanding that defect in an act arises from some defect in the principle of action, some defect in the will must be presupposed before there can be any moral fault. If this defect is natural, it is ever inherent in the will; and the consequence is that the will must always do wrong in action, a consequence proved false by the fact of there being such things as acts of virtue. On the other hand, if the defect is voluntary, that is already a moral fault, the cause of which must stand over for further enquiry; and so we shall have a running account to infinity. We must therefore say that the defect pre-existing in the will is no natural necessity, otherwise it would follow that the will sinned in every act: nor again is a thing of chance and ill luck, for at that rate there could be in us no moral fault, since events of chance are unpremeditated and beyond the control of reason. The defect therefore is voluntary, but not a moral fault: so we must suppose to save the account running to infinity.
Now we must consider how that can be. In every active principle the perfection of its power depends on some superior active principle: for a secondary agent acts by virtue of the power of the prime agent.* So long then as the secondary agent remains under the power of the prime agent, it will act unfailingly: but it will fail in its action whenever it happens to swerve from the order of the prime agent, as appears in an instrument when it ceases to respond to the movement of the agent who uses it. Now it has been said above that in the order of moral actions principles go before volition, the apprehensive faculty and the object apprehended, which is the end in view. But since to everything movable there corresponds a proper motive power, not any and every apprehensive faculty is the due motive power of any and every appetite,* but one apprehension is the proper motive of one appetite, another of another. As then the sensible apprehensive faculty is the proper motive power of the sensible appetite, so the proper motive power of the will is reason itself. Further, as reason can apprehend many sorts of good things and many ends of action; as moreover every power has its own proper end; the will also must have some object and end of action and prime motive, and that must be not any and every sort of good, but some definite good. Whenever then the will tends to act under the motive of an apprehension of reason representing to it its own proper good, a due action ensues. But when the will bursts out into action upon the apprehension of the sensible apprehensive faculty, or even upon the apprehension of reason itself, representing some other good than the proper good of the will, there ensues in the action of the will a moral fault.* Therefore any faulty action in the will is preceded by a lack of due regard to reason and to the proper end of willing. I say 'a lack of due regard to reason,' in such cases as when, upon some sudden aprehension of sense, the will tends to some good that is pleasant according to sense. I say 'a lack of due regard to the proper end of willing,' in cases when the reason arrives by reasoning at some good, which is not either now or in this way good, and still the will tends to it as though it were its proper good. Now this lack of due regard is voluntary: for it is in the power of the will to will and not to will: it is likewise in its power to direct reason actually to consider or to cease from considering, or to consider this or that.* Still this failure of due consideration is not a moral evil: for, consideration or no consideration, or whatever the consideration be on reason's part, there is not sin until the will comes to tend to some undue end, which then is an act of will. -- Thus it remains true that in moral as well as in physical actions, evil is not caused by good except accidentally.
3.8, 9 : Arguments against the aforesaid Conclusion, with Answers to the same
3.11 : That Evil is founded in some Good