1. The principle of double effect must be understood in the light of the teachings of the Magisterium on the general principles that apply to all morality. One common error is to state the principle of double effect only as it was expressed by St. Thomas Aquinas, without regard for subsequent development of doctrine and subsequent definitive teachings of the Magisterium. Another common error is to state the principle of double effect as if it were an exception to the three fonts of morality and the other moral teachings of the Church.
Therefore, in evaluating the principle of double effect, we must include the teachings of Veritatis Splendor on the three fonts of morality and on intrinsically evil acts. It is a serious doctrinal error to interpret or apply the principle of double effect such that any intrinsically evil act would become moral, or would become no longer intrinsically evil, despite having an evil moral object.
2. Another common error is to claim that some acts are morally indifferent or morally neutral, as if any knowingly chosen act could possibly have no moral object at all. This claim is a doctrinal error.
“Freedom makes man a moral subject. When he acts deliberately, man is, so to speak, the father of his acts. Human acts, that is, acts that are freely chosen in consequence of a judgment of conscience, can be morally evaluated. They are either good or evil.” (CCC, n. 1749)
The Catechism of the Catholic Church disagrees with the claim that some acts are morally neutral. An act in moral doctrine is an exercise of free will and intellect, in other words a knowing choice. All knowingly chosen acts are either good or evil.
“no human act is morally indifferent to one’s conscience or before God” (Congregation for Catholic Education)
There is no such thing as a morally indifferent or morally neutral human act. Either an knowingly chosen act is moral, i.e. morally permissible before God and one’s conscience, or that act is immoral and therefore a sin.
What some commentators claim as examples of morally neutral acts are merely an act that is not described with sufficient information to determine its moral object. For example, killing in self-defense has a good moral object; murder has an evil moral object. If we consider the ‘act of killing’ without enough information to determine the moral object, this does not imply that killing is morally neutral. An act in moral theology is a knowing choice; if we are not told what that choice is, we cannot determine the moral object. Each and every knowingly chosen act is intrinsically ordered toward either good or evil as its proximate end; this end is its moral object. If the act is described in such a way that its inherent ordering and its moral object cannot be determined, the fault is in the description. Killing is never morally neutral. Each and every knowingly chosen act of killing is either moral or immoral; its moral object is either good or evil.
3. Another error, actually more of an inaccuracy, is the assertion that, when an action has two effects, one good and the other bad, the bad effect cannot be intended. This assertion is usually part of a list of criteria needed for an act to be moral under the principle of double effect. If all the criteria are met, the act is said to be justified. Since the three fonts of morality always apply, the more accurate statement would be that nothing immoral can be intended. If the person choosing an act has an immoral intention, but not the particular bad intended end of the bad effect, the act is still immoral.
4. A more serious error is the claim that the good effect cannot arise from the bad effect; in other words, the bad effect cannot be the means to the good effect. Certainly, an immoral means is not justified by a good end. If the means is bad in the sense of moral evil, then the choice of that means is a sin, even if it is a means to a good end. However, physical evil is not the same as moral evil. And when we consider the bad consequences of an act, those bad effects are a type of physical evil (harm of any kind, even if it is not literally ‘physical’).
A means that is physical evil only, and not moral evil, can be a moral means to a good end, as long as all three fonts of morality are good. For example, a physician may amputate a limb in order to save a life. The amputation of a limb is physical evil, but it is not moral evil unless it is the intended end, or the moral object. The choice of physical evil as an intended end makes the end morally evil. The choice of an act that is inherently ordered toward physical evil as its proximate end makes the object morally evil. But physical evil can be tolerated, and even intended, as a bad means to a good end, as long as that physical evil is not also moral evil. The physician intends to use the physical evil of amputation as a bad means to a good end. This intention and act is moral because the bad means is not moral evil, but only physical evil, and the good consequence of saving a life outweighs the bad consequence of losing a limb.
By comparison, directly killing an innocent prenatal is not only physical evil (loss of any human life), but also moral evil, the direct and deliberate killing of an innocent human being (murder). Therefore, direct abortion cannot be justified as a bad means to a good end, such as saving the life of the mother. For direct abortion is moral evil, not merely physical evil.
5. The principle of double effect can be summarized in terms of the three fonts of morality taught by Veritatis Splendor, as follows:
INTENTION: The intended end as well as the intended means must be morally good.
MORAL OBJECT: The intentionally-chosen act must be good in itself. An intrinsically evil act is never justified by the principle of double effect. If the moral object is evil, the act is intrinsically evil and always immoral.
CIRCUMSTANCES: The bad consequences of the act must not outweigh the good consequences, in so far as these consequences can be reasonably anticipated at the time that the act is chosen. The consequences are weighed by their moral weight in terms of the love of God and the love of neighbor as self. (The term ‘proportionate’ means that the good consequences are at least of the same moral weight as the bad consequences. The term ‘disproportionate’ means that the bad consequences outweigh the good.)
The commonly-stated additional provision that the bad effect not be the means to the good effect is true whenever the bad effect is morally bad (as when an intrinsically evil act is the means to the good effect), and is true whenever the bad effect, as physical evil, outweighs the good effect. However, the most common phrasing of this provision is erroneous. Physical evil can be tolerated, and even intended, as a means to a good end. The correct understanding of this provision is included in the above three considerations, under (1) that nothing immoral can be intended as a means or an end, and (2) the prohibition against intrinsically evil acts, even as a bad means to a good end, and (3) the need for proportionality in the consequences.