Daniel Hill draws my attention to a Jewish moralist's summary of moral absolutes - things one can never do under any circumstances at all - as a very short list: murder, apostasy, and one or two other things (I think). Can the Catholic tradition produce a similar, neat list?
It looks as though it should be able to. There is a healthy debate on whether lying is forbidden absolutely, or only in certain circumstances. Killing the innocent is clearly prohibited absolutely, as is apostasy. So where's the list?
The problem (if it is a problem) is that Catholic moralists seem to want every prohibition, or at least as many as possible, to be absolute. So, faced with a situation in which a certain class of actions is permissible in some circumstances, and not in others, the tendency is to define a sub-set as being forbidden in all circumstances. So, with lying, for example, we have the taxonomy of officious, jocose and malicious; the last is absolutely forbidden; the jocose lie is not forbidden at all (or at least not on pain of mortal sin), and we can go on arguing about the officious one, but it's probably absolutely forbidden too.
Similarly, to argue that in some cases (the 'starving man and the rich man's surplus') theft is permissible, turns into an argument in favour of a narrower definition of theft, with the starving man case excluded from it, which is absolutely forbidden.
Is this a good or a bad methodological tendency? Although it seems confusing, I think it is the inevitable consequence of two good things. First, it derives from an attempt to cover every case in a clear way. Jewish casuistry is equally interested in hard cases as Catholic casuistry, but is less interested (as far as I can see) in reducing the casuistical analysis to a statement of principles. (Ie to say: now we've got a set of cases where the act is wrong and a set where it is right, what exactly is the principle dividing them?) With the ultimate focus on specific morally relevant principles, rather than more general principles with a set of exceptions, you end up with more moral absolutes.
Second, the role of intention (as usual!) has a role here. The concept of intention makes it possible to produce these plausible and specific moral principles, since the intention of each kind of harm to another is something which can be absolutely forbidden. The exceptions to rules such as 'do not kill' mostly turn out to be cases in which the killing is not intended.
In dealing with non-intended harms there is, instead of an absolute prohibition, the principle of proportionality.
In conclusion, one could say (although the manualists don't talk like this) that there is just one absolute prohibition in Catholic ethics: do not intend harm. And one non-absolute restriction: do not cause foreseen and disproportionate harm. (Plus there are positive duties, such as the duty of worship and obedience to God, and the duty of aid to our neighbour.) Since the question of what is 'harm' in the necessary sense is a controversial one, it is more usual to specify the absolute restrictions as: do not intend deaths, losses of property, etc., each of which has a sister-principle limiting non-intended harms of that kind by the principle of proportionality.