I answer that, As stated above (90, A1,2), law is framed as a rule or measure of human acts. Now a measure should be homogeneous with that which it measures, as stated in Metaph. x, text. 3,4, since different things are measured by different measures. Whereforelaws imposed on men should also be in keeping with their condition, for, as Isidore says (Etym. v, 21), law should be "possible both according to nature, and according to the customs of the country." Now possibility or faculty of action is due to an interior habit or disposition: since the same thing is not possible to one who has not a virtuous habit, as is possible to one who has. Thus the same is not possible to a child as to a full-grown man: for which reason the law for children is not the same as for adults, since many things are permitted to children, which in an adult are punished by law or at any rate are open to blame. In like manner many things are permissible to men not perfect in virtue, which would be intolerable in a virtuous man.
Now human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of others, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like.
Could a case be made that abortion should be tolerated, because to prohibit it would lead to greater evils? (Moral evils, or just evils as in bad consequences?) And abortion is not of the same gravity as murder, in so far as it does not undermine society (peace and relations between citizens) as much as murder does?
Is there a difference between directly legislating "a right to abortion" and merely tolerating it? Is it correct to say that Roe v. Wade establishes this right through the right to privacy
If we take Aquinas as a guide, it would seem that the gravity of abortion is enough that it should be prohibited, and that it is something that can be avoided by the majority. As for any bad consequences that might arise from banning it, I do not see how they outweigh the injustice that is done through abortion.
Pope Benedict XVI and Pope John II have said that abortion is both a crime against the child and against society, and it undermines social life. I do wonder if the social impact is as great as murder--there is the subjective culpability of the women who have abortions to take into consideration, as well as their recognition (or lack of) of the humanity of the conceptum. Abortion does not seem to destroy peace in the same way as murder--though its acceptance can further the decline and eventual self-destruction of society. It does not seem to be an exaggeration to say that lust can have this much of an effect.
