The object is the good which is chosen. Of course, all that God has made is good. Thus, even our sins could be seen as the choosing of a lesser good in the place of a greater good. The object of an action is external to the person choosing it. In other words, the object of the act can be in itself good or evil (cf. CCC 1751).
The intention is in the person acting and helps us evaluate if an act is good or evil by determining the end towards which someone is aiming by acting. However, if an object deliberately chosen is bad, then no intention can make it a good act. On the same token, you may have an objectively good act, such as almsgiving, which is subverted by a bad intention such as vanity (cf. CCC 1752-3).
The circumstances, including the consequences of the act, are secondary to object and intention. The circumstances may increase or diminish thgoodness or evil of human acts. They cannot change the quality of the moral act to make it good or evil (cf. CCC 1754).
This brings us to our topic. What is the Principle of Double Effect? First, we will discuss the moral tradition itself. Then, we will briefly say what it is not.
The Principle of Double Effect comes into play when a given moral act has both a good effect and an evil effect. This principle allows us to determine if such an act may be chosen without falling into sin. Primarily, St. Thomas Aquinas articulates this point to demonstrate that killing is self-defense can be justified.
The New Catholic Encyclopedia states the conditions for the application of the Principle of Double Effect in this way:
The object of the act, which is external to the person acting, must be good or at least indifferent. This is because no amount of good intention can make an evil act good. And because the effect of an act belongs to the circumstances, it cannot make a good act evil or an evil act good.
In other words, the ends never justify the means. The object of the act cannot ever be evil or it is sinful to be chosen. For example, deliberate and willful murder as the deliberate killing of an innocent human person will never be an acceptable action. Likewise, rape and adultery are never justified by intention or circumstances. We can call these intrinsic evils.
Permitting the bad effect is a toleration, not an endorsement. If it is foreseen that there is another course of action which would achieve the good effect without causing the bad effect, then the person would be obliged to choose that course of action. By choosing a good or indifferent act with a good and bad effect, the bad effect stands next to the good effect, it is not being deliberately chosen.
Again, it must be stated, in Catholic theology, the ends can never justify the means. You cannot choose an evil act in order to achieve a good effect. Truly, God will always draw a good out of every evil, but woe to us if we are the author of the evil.
The good effect and bad effect must happen at the same time or, at least, be caused by the action itself. In other words, the good effect cannot be a direct result of the bad effect. If that is the case, then the bad effect is actually an evil act that is being chosen and the Principle of Double Effect does not apply.
There must be a very serious reason that the evil effect is tolerated. The good effect that is directly intended must be significantly more dire to attain than the bad effect that is accepted. Again, we are assuming that there is no other way to achieve the gravely necessary good that is desired.
There are two main areas that Pope St. John Paul II warns about in his masterwork Veritatis Splendor (Splendor of Truth): consequentialism and proportionalism.
Consequentialism seeks to determine if an act will do a maximal amount of foreseeable good. This theory of morality is wrong because it allows for evil acts to be chosen in anticipation of a good effect for a maximal number of people. This falls under intention and circumstance, which cannot effect the object of the act, which is always good, evil, or neutral. If an evil object is chosen, then it is an evil act, no matter how much good is eventually drawn from it.
Proportionalism weighs the various goods and values being sought and then focuses on the proportion between the good and bad effects of that choice, which leads to a “greater good” or “lesser evil” outcome.
Both consequentialism and proportionalism ultimately fail because it is not possible to glean from them any notion of an intrinsically good object or an intrinsically evil object. Yet, there are certain kinds of behavior which are always evil or always good, regardless of time, place, culture, people, intention, and circumstance.