"Necessity" does not canonically excuse intrinsically evils acts like abortion
1. Doyle observes that canon law recognizes the "doctrine of necessity", a jurisprudence whereby some actions (or omissions) that, under most circumstances, would be illegal and punishable, may be performed (or omitted) with impunity. Doyle correctly cites 1983 CIC 1323-1324, their predecessor-in-part 1917 CIC 2205, and two classic canonical commentaries (Michiels and Wernz-Vidal) to show that canon law recognizes the "doctrine of necessity".
That's all fine, though no one I know disputes that canon law (like every other legal system worthy of the name) has a "doctrine of necessity". The question is rather when and how "necessity" can be plead. Now, to get right to my point, "necessity" cannot be plead as exculpatory* of an intrinsically evil action like abortion. In demonstration whereof, I can cite the same authorities invoked by Doyle, Michiels and Wernz-Vidal.
Michiels (five pages past the passage quoted by Doyle) writes: "necessity [under Canon 2205] is never said to excuse from intrinsically evil acts prohibited by the natural law itself," and, just a couple pages later, Michiels explicitly lists abortion as an act whose intrinsically evil character prevents "necessity" from being urged as an excuse. Wernz-Vidal (two pages past the passage quoted by Doyle), agree: "nor does this principle [of necessity under Canon 2205] apply to things illicit in natural law, things which cannot be rendered licit by any necessity." It would be easy to cite to numerous other canonists and moral theologians who reject, without exception, the intentional performance of an evil action to achieve any good, but Doyle's own sources, when read more completely, make the argument sufficiently.
Evidently, "necessity" defenses are not to be lightly invoked in penal canon law.
2. I am sure that Doyle does not want to challenge Church teaching that deliberate abortion committed against a human being is an intrinsic evil (CCC 2271 etc.), so I would assume that his moral advice to the faithful contemplating such a horrific idea would be to never cooperate with or consent to it, regardless of the appealing rationales offered for the deed or the ambiguities lurking in imperfectly phrased laws.
*Scholion on 1983 CIC 1324.1.5. What does one make of language in the 1983 Code that suggests "necessity" as some of level of defense even to intrinsically evil acts? Some thoughts: (1) unbroken moral tradition against doing any evil to achieve good trumps the phraseology of a canon that, in a number of respects, seems poorly worded; (2) this anomalous "necessity" language does not appear in Eastern canon law; (3) 1983 CIC 1324 and 1917 CIC 2205 permit "necessity" to be plead only in mitigation of liability for intrinsically evils acts, but never in exculpation; and (4) the facts giving rise to a "necessity" plea in the first place would likely diminish culpability on other grounds (e.g., force and fear, or the relative inferiority of the violated law) and so are probably better invoked under those headings. In short, while I would clarify Canon 1325 in regard to necessity, I would not, in the meantime, advise clients that, absent such clarification in the law, they may commit canonical crimes that are intrinsically evil provided that the good they hope to achieve thereby is, by some characterization of necessity, sufficiently impressive.
See also my May 21, 2010 post, "The Phoenix abortion case"