Bernard Hoose's Proportionalism
Aquinas' Natural Law is both teleological and deontological. The Primary Precepts are the end or purpose (telos) of humans. They lead to secondary (deontological) precepts. Natural Law is generally seen as deontological because it largely consists of absolute laws such as 'Abortion is wrong', 'No euthanasia' etc.
Bernard Hoose agrees with the rules of Natural Law - a foetus should grow into a baby, and a sick person should be treated not killed. However, Hoose believes there are exceptions to the rules. For example, if a pregnancy will kill both the foetus and the mother, it seems obvious that in these circumstances you should break the rules and abort the foetus.
Proportionalism can be seen as a development of Natural Law. In Natural Law, certain acts are intrinsically evil, such as lying or stealing. However, the intention is important. For example, if a pregnancy is ectopic (the foetus is growing in the fallopian tube), you can't kill the foetus to save the mother, as killing is intrinsically evil. However, you can remove the fallopian tube to save the mother, even if it leads to the foetus dying (as a 'double effect').
Hoose believes the act of killing the foetus in this case would be an 'ontic evil', not a 'moral evil'. If killing the foetus will save the mother, and the mother can still have children, it doesn't make sense to remove the fallopian tube instead.
An example of an ontic evil is removing someone's heart. It is clearly wrong to remove someone's heart, but before we can say whether it's morally wrong, we need to know the individual circumstances. If a surgeon is carrying out a heart transplant to save someone's life, this may be morally right. As such, the rules of Natural Law are good rules, and it is wrong to lie, steal or kill, but in some individual cases where there is a proportionate reason to break these rules, it may not be morally wrong to do so.
Proportionalism is considered to be a hybrid between Situation Ethics and Natural Law. As such, it may be considered to have the positive aspects of each theory - the rules of Natural Law are a useful guide to morality, whereas the flexibility of Situation Ethics allows you to break the rules if there is a good reason to do so.