Many arguments regarding abortion or euthanasia (or assisted dying more generally) are based on supposed “laws of nature.” The status of such natural laws is not clear, but we are assured that they are, notwithstanding their nature as facts about the world, morally binding upon those to whom they apply. We saw this in action not so long ago when I addressed myself to Edward Feser’s book The Last Superstition. One of the things that struck me with such force about that book, and the response to my criticism of it — and here it is important to remember that the entire milieu of contemporary scholastic philosophy, basing itself on Aristotle and Aquinas, is related as like to unlike when it is used in philosophical discussion itself — is that there is a sense in which, for the modern scholastic, the arguments are already made and the conclusions are already reached, well before we begin, and there is nothing, beyond simple agreement, that can be accepted as the rational response to scholastic argumentation.
This is particularly clear in Robert George’s book, The Clash of Orthodoxies. When he is discussing Rawls’ theory of justice, he takes it for granted that the idea of justice as fairness is all very well where we do not know the truth, but where we do know it – as, he claims, we would know it if we accepted his argumentation, for this argumentation produces truth of such power and validity that it cannot be reasonably questioned by a rational person — any settlement made behind the “veil of ignorance” (or, as Rawls alternatively says, in the “original position”) is immediately called into question by any rational demonstration of the truth. George believes himself to have established, beyond reasonable doubt, that abortion, infanticide and euthanasia or assisted suicide (assisted dying) are contrary to the natural law, and therefore irrational and immoral. This, it would seem, immediately upsets the possibility of establishing an “original position” (even as a philosophical hermeneutic) from which political arrangements can be made which will assure the just (=fair) disposition of rights and privileges, wealth and opportunity that is consistent with the greatest benefit to the least well off person in society. For if justice, established in this way, must be subject to the truth (as understood by a subsection of the citizenry), any such contractual arrangements would be immediately called into question by the rational demonstration that something believed, behind the veil of ignorance, to be a matter of individual choice and determination, is, in actuality, not open to reasonable question.
The important point to notice here is that Rawls’ theory of justice was intended to overcome the ideological differences between different members of society, so that, for those, like George, who believe that their position is the only rational one, in terms of providing conclusive argument for the truth of its conclusions, there would be no hindrance to their believing themselves to have achieved truth, and to act in accordance with it, without at the same time requiring others to subordinate their own lives or reasoning to the conclusions reached by George and his fellow believers. When Rawls speaks of comprehensive world views existing side-by-side in the same society on the basis of contractual agreement not to impose views peculiar to themselves on others — thus preserving both peace and justice in society — he has in mind claims like those made by George, that their arguments offer conclusive demonstrations of the truth of their beliefs. In the kind of secular society for which Rawls’ theory of justice was intended as a means of achieving social peace and concord, as well as providing the greatest liberty for each citizen compatible with equal liberty for all, the move that George is making is simply illegitimate, for it is claiming moral certainty in a world where no such certainty exists.
Let’s go back to Edward Feser for a moment. In his book, The Last Temptation, he argues that Terri Schaivo,
since she was still alive and thus still had a rational soul, [she] was no less a rational animal than her husband and the judges who together condemned her to death, even if, unlike them, she could not exercise her rationality. 
Depriving her of food therefore, in Feser’s view, constituted murder. And this is, in fact, how the Roman Catholic Church continues to regard the decision to allow Terri Schaivo to die. On the sixth anniversary of her death, Alex Schadenberg put the following on his blog:
We remember Terri Schiavo
Six years ago, today, Terri Schiavo died of dehydration (slow euthanasia). This is called slow euthanasia because Terri was not terminally ill, and she only needed basic care, fluids, food and love to live. Her family wanted to provide that care but her husband wanted her dead.
We remember Terri. Terri was dehydrated with the intention of causing her death. She directly died from dehydration. We will never forgot.
The appearance of a smile on Terri Schaivo’s face in this picture — the one most often reproduced — is almost certainly not a smile, but is more likely caused by the involuntary flexing of the facial muscles.
The belief that unfortunates like Terri Schaivo have a rational soul, and are, with respect to the law, no different from other human beings, is belied on the very next page, where Feser suggests that
[s]ince it is the form of a rational animal, the matter a rational soul informs must be complex enough to sustain those material operations that it relies on in an indirect way, such as sensation. [131, my emphasis]
The rational soul itself, however, must come, Feser believes, from outside the natural process of evolution, though evolution is not, he says, completely irrelevant. Of course, in the case of Terri Schaivo, the matter of her brain was in fact not complex enough — because of brain damage caused by cardiac arrest thought to have been brought on by a potassium imbalance caused by her bulimia – to support thought or sensation, or even the ‘ordering toward the knowledge of God’ (which is, we are told, the special function of the rational soul). Of course, the language of ‘relied on in an indirect way’ is merely a doctrinal point, and is not the result of empirical knowledge or investigation.
This immediately raises the question of the meaning of the word ‘nature’ in the expression ‘natural law’ as in ‘natural law morality’. For instance, in the article by Judith Jarvis Thomson, “A Defensive of Abortion” (Philosophy and Public Affairs, vol. 1, no. 1 (Autumn, 1971), pp. 47-66), to which Robert George refers, George says (in a footnote), in response to Thomson:
Thomson’s rhetoric here, referring to “Catholic doctrine” on abortion, helps her case along by presenting the pro-life position, at least as it figures as part of Catholic moral teaching, as a sectarian matter that relies on religious premises that are somehow unavailable to non-Catholics. The “Catholic doctrine” on the subject, however, condemns abortion as homicidal and unjust as a matter of publicly accessible scientific fact and rational (natural law) morality. [329, footnote 39, italics mine]
However, this does not help, but it does highlight Hume’s point when he made the observation that, whenever he read moral philosophy the language used modulates, almost imperceptibly, from matters of fact, using such words as is and is not, to moral prescriptions, using such words as ought and ought not (see A Treatise of Human Nature, Book 3, Section 1, Para 27). The problem is, what have accessible scientific facts to do with rational (that is, natural law) morality?
What kinds of natural laws are these? They do not seem in any sense to be scientific laws of nature, such as the inverse square law pertaining to the attraction of masses to each other, yet George appeals to them as though they had the same kind of weight and significance. In his essay “On Suicide” Hume mocks the idea of natural law, by suggesting that since it would not be thought a crime
to divert the Nile or Danube from its course, were I able to effect such purposes. Where then is the crime of turning a few ounces of blood from their natural channels [in dying by suicide]?
Tom Beauchamp, in a critique of Hume’s essay (An Analysis of Hume’s Essay “On Suicide”. The Review of Metaphysics, vol. 30, no. 1 (September, 1976), pp. 73-95), suggests that this mockery is based on Hume’s misunderstanding of the idea lying behind natural law morality. For, while laws of nature, such as Newton’s inverse square law, are descriptive accounts of universal regularities in nature,
natural [moral] laws do not empirically describe behavior but rather delimit the behavior which is morally appropriate for a human being.
He also points out, further, that
[n]atural laws are the orderings of potentialities to their actualizations, and human goods are determined by reference to these orderings.
However, I think that Hume probably knew well enough what Aquinas was getting at in speaking of natural moral laws, because Aquinas did in fact base natural moral laws on perceived regularities in nature. For instance, Aquinas quite clearly holds that the prohibition of suicide is based precisely on the fact that, by nature,
everything … loves itself, the result being the everything naturally keeps itself in being, and resists corruptions so far as it can. [II-II, Q. 64, Art 5]
This is an empirical description of nature, and is as much a law of nature, in Aquinas’ account, as is the fact that water naturally flows downhill. Beauchamp’s problem is more serious than he thinks. For not only does Hume’s understanding of natural law accord well with that of Aquinas, who borrowed his understanding from Aristotle, the truth is that using the Aristotelian language of potentiality and actuality in relation to morality, and distinguishing this understanding of natural law from the scientific understanding of the laws of nature is deeply misleading. The language of natural law morality may stick quite closely to Aristotelian science, but now that we understand natural law or the laws of nature in a very different way, it makes little sense to go on using the language of natural law in relation to morality, for there is no reason to make the kinds of distinction between natural laws and laws of nature to which Beauchamp appeals in his criticism of Hume. Hume is showing, quite decisively, it seems to me, that the logical outcome of continuing to use this language is nonsense.
I conclude that Hume’s mocking of natural law morality is perfectly legitimate, for what it shows so clearly is that it is no longer appropriate to speak about human actions in Aristotelian terms. The only laws of nature that there are, are general empirical descriptions of the causal relationships of natural things (at whatever dimension), confirmed by observation and experiment. So when Hume speaks of the idea of natural laws as laid down by God in creation “reserved as the peculiar province of the Almighty,” and goes on to point out that, on this reading, if it is “an encroachment on [God's] right for men to dispose of their own lives, it would be equally criminal to act for the preservation of life as for its destruction,” (my italics) he is quite reasonably equating laws of nature with natural law, and deriving nonsense from the equation.
John Donne recognised the same thing. In his book on suicide, Biathanatos, John Donne put it in the following way. He is speaking about sin, about word, deed and desire opposed to the eternal law of God (“dictum, factum, concupitum contra aeternam legem Dei“), and, as he says, this cannot pertain only to those things recognised as sinful, for, he says,
a man may, without sin, both think and speak and do, as I may resist a disease of which God hath decreed that I shall die. Yea, though He seem to reveal His will, we may resist it with prayers against it, because it is often conditioned and accompanied with limitations and exceptions. [Biathantos, A modern spelling edition, 1982, p. 51]
The point, quite simply, is that natural law cannot do what it is supposed to do, namely, to place limits upon what we can reasonably do, and that any action may be taken to be contrary to natural law in this sense, even actions which are not in any sense that we can understand, morally wrong. However, if acting contrary to natural law is sinful, as Aquinas assumes, then preserving one’s life when it is God’s purpose to end it would be as sinful as taking one’s life when it is God’s will to preserve it.
Thus, when Judith Jarvis Thomson quotes from Pius XI’s Encyclical Casti Connubii, in response, it is interesting to note, to a decision of the Lambeth Conference of Anglican bishops meeting in 1930, which gave limited support to artificial means of birth control, to the effect that
however much we may pity the mother whose health and even life is gravely imperilled in the performance of the duty allotted to her by nature, [op. cit., 51, fn 4]
the words ‘the duty allotted to her by nature’ are no more legitimately morally prescriptive than are Newton’s laws of motion. So that Thomson can say, with considerable reason, that
It cannot seriously be thought to be murder if the mother performs an abortion on herself to save her life. It cannot seriously be said that she must refrain. 
This, however, is what is being said with the greatest seriousness by people like Pope Pius XI and Robert George, and it is hopeless to claim that it cannot seriously be said. What we can say is that it would be obscene to make this suggestion seriously, as it is. And yet it is upon this thin thread that Robert George’s rational morality dangles precariously.