The foregoing arguments enable us to conclude that evil is caused only by the good.
 For, if an evil thing were the cause of a certain evil, then the evil thing would not act, except by virtue of the good, as has been proved. So, this good must be the primary cause of the evil.
 Again, what does not exist is not the cause of anything. So, every cause must be a definite thing. But evil is not a definite being, as has been proved. Therefore, evil cannot be the cause of anything. if, then, evil be caused by anything, this cause must be the good.
 Besides, whatever is properly and of itself the cause of something tends toward a proper effect. So, if evil were of itself the cause of anything, it would tend toward an effect proper to it; namely, evil. But this is false, for it has been shown that every agent tends toward the good. Therefore, evil is not the cause of anything through evil itself, but only accidentally. Now, every accidental cause reduces to a cause that works through itself. And only the good can be a cause through itself, for evil cannot be a cause through itself. Therefore, evil is caused by the good.
 Moreover, every cause is either matter, or form, or agent, or end. Now, evil cannot be either matter or form, for it has been shown that both being in act and being in potency are good. Similarly, evil cannot be the agent, since anything that acts does so according as it is in act and has form. Nor, indeed, can it be an end, for it is apart from intention, as we have proved. So. evil cannot be the cause of anything. Therefore, if anything is the cause of evil, it must be caused by the good.
 In fact, since evil and good are contraries, one of these contraries cannot be the cause of the other unless it be accidentally; as the cold heats, as is said in Physics VIII [1: 251a 33]. Consequently, the good could not be the active cause of evil, except accidentally.
 Now, in the order of nature, this accidental aspect can be found either on the side of the agent or of the effect. It will be on the side of the agent when the agent suffers a defect in its power, the consequence of which is a defective action and a defective effect. Thus, when the power of an organ of digestion is weak, imperfect digestive functioning and undigested humor result; these are evils of nature. Now, it is accidental to the agent, as agent, for it to suffer a defect in its power; for it is not an agent by virtue of the fact that its power is deficient, but because it possesses some power. If it were completely lacking in power, it would not act at all. Thus, evil is caused accidentally on the part of the agent in so far as the agent is defective in its power. This is why we say that “evil has no efficient, but only a deficient, cause,” for evil does not result from an agent cause, unless because it is deficient in power, and to that extent it is not efficient.—And it reduces to the same thing if the defect in the action and in the effect arise from a defect of the instrument or of anything else required for the agent’s action; for example, when the motor capacity produces lameness because of a curvature of the tibia. For the agent acts both by means of its power and of its instrument.
 On the side of the effect, evil is accidentally caused by the good, either by virtue of the matter of the effect, or by virtue of its form. For, if the matter is not well disposed to the reception of the agent’s action on it, there must result a defect in the product. Thus, the births of monsters are the result of lack of assimilation on the part of the matter. Nor may this be attributed to some defect in the agent, if it fail to convert poorly disposed matter into perfect act. There is a determinate power for each natural agent, in accord with its type of nature, and failure to go beyond this power will not be a deficiency in power; such deficiency is found only when it falls short of the measure of power naturally due it.
 From the point of view of the form of the effect, evil occurs accidentally because the privation of another form is the necessary concomitant of the presence of a given form. Thus, simultaneously with the generation of one thing there necessarily results the corruption of another thing. But this evil is not an evil of the product intended by the agent, but of another thing, as was apparent in the preceding discussion.
 Thus it is clear that, in the natural order, evil is only accidentally caused by the good. Now, it works in the same way in the realm of artifacts. “For art in its working imitates nature,” and bad results occur in both in the same way.
 However, in the moral order, the situation seems to be different. It does not appear that moral vice results from a defect of power, since weakness either completely removes moral fault, or at least diminishes it. Indeed, weak ness does not merit moral punishment that is proper to guilt, but, rather, mercy and forgiveness. A moral fault must be voluntary, not necessitated. Yet, if we consider the matter carefully, we shall find the two orders similar from one point of view, and dissimilar from another. There is dis similarity on this point: moral fault is noticed in action only, and not in any effect that is produced; for the moral virtues are not concerned with making but with doing. The arts are concerned with making, and so it has been said tha in their sphere a bad result happens just as it does in nature Therefore, moral evil is not considered in relation to the matter or form of the effect, but only as a resultant from the agent.
 Now, in moral actions we find four principles arranged in a definite order. One of these is the executive power, the moving force, whereby the parts of the body are moved to carry out the command of the will. Then this power is moved by the will, which is a second principle. Next, the will is moved by the judgment of the apprehensive power which judges that this object is good or bad, for the objects of the will are such that one moves toward attainment, another moves toward avoidance. This apprehensive power is moved, in turn, by the thing apprehended. So, the first active principle in moral actions is the thing that is cognitively apprehended, the second is the apprebensive power, the third is the will, and the fourth is the motive power which carries out the command of reason.
 Now, the act of the power that carries out the action already presupposes the distinction of moral good or evil. For external acts of this kind do not belong in the moral area, unless they are voluntary. Hence, if the act of the will be good, then the external act is also deemed good, but if it be bad, the external act is bad. It would have nothing to do with moral evil if the external act were defective by virtue of a defect having no reference to the will. Lameness, for instance, is not a fault in the moral order, but in the natural order. Therefore, a defect of this type in the executive power either completely excludes moral fault, or diminishes it. So, too, the act whereby a thing moves the apprehensive power is free from moral fault, for the visible thing moves the power of sight in the natural order, and so, also, does any object move a passive potency. Then, too, this act of the apprehensive power, considered in itself, is without moral fault, for a defect in it either removes or diminishes moral fault, as is the case in a defect of the executive power. Likewise, weakness and ignorance excuse wrongdoing, or diminish it. The conclusion follows, then, that moral fault is found primarily and principally in the act of the will only, and so it is quite reasonable to say, as a result, that an act is moral because it is voluntary. Therefore the root and source of moral wrongdoing is to be sought in the act of the will.
 However, a difficulty seems to result from this investigation. Since a defective act stems from a defect in the active principle, we must understand that there is a defect in the will preceding the moral fault. Of course, if this defect be natural, then it is always attached to the will, and so the will would always commit a morally bad action when it acts. But virtuous acts show that this conclusion is false. On the other hand, if the defect be voluntary, it is already a morally bad act, and we will have to look in turn for its cause. Thus, our rational investigation will never come to an end. Therefore, we must say that the defect pre-existing in the will is not natural, to avoid the conclusion that the will sins in everyone of its acts. Nor can we attribute the defect to chance or accident, for then there would be no moral fault in us, since chance events are not premeditated and are beyond the control of reason. So, the defect is voluntary. Yet, it is not a moral fault; otherwise, we should go on to infinity. How this is possible we must now explain.
 As a matter of fact, the perfection of the power of every active principle depends on a higher active principle, since a secondary agent acts through the power of a primary agent. While, therefore, a secondary agent remains in a position of subordination to the first agent, it acts without any defect, but it becomes defective in its action if it happens to turn away from its subordination to the primary agent, as is illustrated in the case of an instrument, when it falls short of the motion of the agent. Now, it has been said that two principles precede the will in the order of moral actions: namely, the apprehensive power, and the object apprehended, which is the end. Since to each movable there corresponds a proper motive power, not merely any apprehensive power is the suitable motive power for any and every appetite; rather, one pertains to this appetite and another to a second appetite. Thus, just as the proper motive power for the sensory appetite is the sensory apprehensive power, so the reason itself is the proper motivator for the will.
 Again, since reason is able to apprehend many goods and a multiplicity of ends, and since for each thing there is a proper end, there will be, then, for the will an end and a first motivating object which is not merely any good, but some determinate good. Hence, when the will inclines to act as moved by the apprehension of reason, presenting a proper good to it, the result is a fitting action. But when the will breaks forth into action, at the apprehension of sense cognition, or of reason itself presenting some other good at variance with its proper good, the result in the action of the will is a moral fault.
 Hence, a defect of ordering to reason and to a proper end precedes a fault of action in the will: in regard to reason, in the case of the will inclining, on the occasion of a sudden sense apprehension, toward a good that is on the level of sensory pleasure; and in regard to a proper end, in the case when reason encounters in its deliberation some good which is not, at this time or under these conditions, really good, and yet the will inclines toward it, as if it were a proper good. Now, this defect in ordering is voluntary, for to will and not to will lie within the power of the will itself. And it is also within its power for reason to make an actual consideration, or to abstain from such a consideration, or further to consider this or that alternative. Yet, such a defect of ordering is not a moral evil, for, if reason considers nothing, or considers any good whatever, that is still not a sin until the will inclines to an unsuitable end. At this point, the act of will occurs.
 Thus, it is,car, both in the natural order and in the moral order, that evil is only caused by good accidentally.
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