In recent decades, medical science has made significant strides in understanding human life in its initial stages. Human biological structures and the process of human generation are better known. These developments are certainly positive and worthy of support when they serve to overcome or correct pathologies and succeed in re-establishing the normal functioning of human procreation. On the other hand, they are negative and cannot be utilized when they involve the destruction of human beings or when they employ means which contradict the dignity of the person or when they are used for purposes contrary to the integral good of man.

The body of a human being, from the very first stages of its existence, can never be reduced merely to a group of cells. The embryonic human body develops progressively according to a well-defined program with its proper finality, as is apparent in the birth of every baby.

It is appropriate to recall the fundamental ethical criterion expressed in the Instruction Donum vitae in order to evaluate all moral questions which relate to procedures involving the human embryo: "Thus the fruit of human generation, from the first moment of its existence, that is to say, from the moment the zygote has formed, demands the unconditional respect that is morally due to the human being in his bodily and spiritual totality. The human being is to be respected and treated as a person from the moment of conception; and therefore from that same moment his rights as a person must be recognized, among which in the first place is the inviolable right of every innocent human being to life".

(Congregation for the Doctrine of the Faith, "Instruction Dignitas Personae on Certain Bioethical Questions" [8 September 2008], pt. 1, sec. 4 [italics in original; boldface added; footnote omitted])

Note from KBJ: The key words are "inviolable" and "innocent." The word "inviolable" makes the prohibition absolute, i.e., exceptionless. If the word "innocent" were omitted, then there would be an absolute prohibition on killing, from which it would follow that (1) it is impermissible to kill in self-defense, (2) it is impermissible to kill convicted murderers, and (3) it is impermissible to kill combatants even in a just war. The Catholic church rejects these propositions. If we define "murder" as the killing of an innocent human being and "homicide" as the killing of a human being, then there is an absolute prohibition on murder—but not on homicide. See the difference?

Note 2 from KBJ: Here is a long blog post about the Church's position on capital punishment. If you want to read just one item on the topic, I recommend this, by the late cardinal Avery Dulles. Key paragraph:

The Catholic magisterium does not, and never has, advocated unqualified abolition of the death penalty. I know of no official statement from popes or bishops, whether in the past or in the present, that denies the right of the State to execute offenders at least in certain extreme cases. The United States bishops, in their majority statement on capital punishment, conceded that "Catholic teaching has accepted the principle that the State has the right to take the life of a person guilty of an extremely serious crime." Joseph Cardinal Bernairdin [sic], in his famous speech on the "Consistent Ethic of Life" at Fordham in 1983, stated his concurrence with the "classical position" that the State has the right to inflict capital punishment.

I'd be interested in hearing from my Catholic readers. If you're reading this, Jeff H., write to me.