The effect of an action, the result or product of an effective cause or agency, may in itself be an end or an object or a circumstance, and it has influence in the determination of morality. Sometimes an act has two effects, one good and the other bad; and that such an action be lawful it is necessary (1) that the action itself be good or indifferent; (2) that the good effect be intended and the evil effect be not intended (chosen) but only reluctantly permitted; (3) that the evil effect be not a means to secure the good effect; (4) that there be present a motive sufficiently grave to excuse or counterbalance the bad effect. {15} St. Thomas (_Sum. Theol_. 2. 2. q. 64, a. 7) Speaking of killing a man in self-defence, says: "Nihil prohibet unius actus esse duos effectus, quorum alter solum sit in intentione, alius vero sit praeter intentionem. Morales autem actus recipiunt speciem secundum id quod intenditur, non autem ab eo quod est praeter intentionem, cum sit per accidens." That an act, therefore, be morally good, or justifiable, (a) the whole train of the tendency of the will must be good; that is, (1) the object, (2) the end, (3) and the circumstances must be good; or (b) the intention should be good, and the remaining elements in the train of will-tendency are to be indifferent. That an act be morally bad it is enough that the object, the end, or the circumstances be inexcusably bad. There may be honest doubt as to the existence of evil in the circumstances or the end, and here enters the matter of probability; but apart from this, some general rules of morality that govern all cases may be formulated: 1. An intention or end which is gravely evil always makes the entire action evil and unjustifiable. 2. An intention or end which is slightly evil, if it is the entire end of an action, makes the whole action evil but not gravely evil--makes it, say, a venial sin and not a mortal sin. 3. If an intention or end which is venially evil accompanies secondarily a good intention or end, and is rather a motive than the real effective agent in attracting the will, this venial evil does not vitiate the whole goodness or righteousness of the main action. Compare the remarks made above in discussing an action that has a double effect, partly good and partly bad. 4. Circumstances that are gravely evil practically vitiate the entire action, but circumstances which are venially evil do not always vitiate the entire action.