A moral code is primarily of use in telling me how I should act. It may also be possible to use it when evaluating the actions of others. Aquinas' "Doctrine of Double Effect" (DDE) says that "moral acts are categorized in accordance with what is intended", and that unintentional side effects are permissible, so long as they are not "out of proportion".

So far as my own morality is concerned, I at least know my own intentions, and it seems that I should therefore be able to judge whether I am acting morally. The main problem with the DDE is that, as stated, it is of little help in determining which unintended side effects are "out of proportion" to good ones. Aquinas says that killing an attacker in order to save my own life is permissible, but it's unlikely I'm going to encounter that situation: the moral dilemmas we face in real life are a lot less straightforward.

In the case of other people, it is impossible for me to know what their intentions are, and so I cannot determine with certainty whether another person has acted morally or not. [However, we do commonly use something very like the DDE in judging the morality others: the legal system may charge a man with "murder", "manslaughter", "accidental killing", or "attempted murder" depending on the results of his actions and his (presumed) intention.]

Additionally, when it comes to the actions of others, it does seem that the allowable "foreseen consequences" clause could be used as an excuse for just about anything. Cigarette manufacturers might accept that a consequence of people smoking is that they will suffer ill-health, but argue that this is an unintended consequence, and so not something for which they are culpable.

However, the DDE does seem to articulate what most people would probably regard as a reasonable standard for moral behaviour: that you have acted morally provided you meant well, and that your actions didn't have disproportionate side-effects. In this, I think Aquinas' doctrine is reminiscent of Ross' writing on "prima facie duties": it appears that he has tried to construct a moral code based on a generally agreed upon set of social conventions, rather than using an a priori approach.