Classical Theism

1. Goodness is convertible with being (Things have to be fully actualized according to their nature to be fully good)
2. Objective goodness exists.
3. In a non-natural law environment objective goodness is arrived at best if we follow our conscience.
4. But our conscience is not a universal guide. People disagree about what is intuitively good.
5. Because of 4, 3 is not a good guide to arrive at 2.
6. Hence we should accept 1 to arrive at 2.
7. Therefor we should accept natural law.

The weakness in natural law might be that we don´t fully know the essence of a thing or all the goals of a particular faculty. But for example when we run with the example of abortíon it is only 1 that leaves us with direct arguments from nature why it should be considered an immoral act. It is also only 1 that gives direct arguments as to why the life is to be considered more important than the choice. Arguing from objective morality to the other side, I don´t see how that is possible without invoking abstract rules that lack an intelligible foundation.
Natural law is not perfect, but I think it is what provides the best basis from which ethics can be developed.

I too am interested in why one should follow natural law ethics. Also, if someone could explain the difference between the "old" and "new" natural law, I am curious about that also.

@T N

When I say things like good, I am appealing to my intuitive sense of what good is. Certainly, I do not always use the term good in the same way that a natural law theorist does. For example, I think that it is good that a husband and a wife have sex, with or without the use of various types of contraception. This would be rejected on an analysis of a natural law theorist.

I am also using the term good in an ought sense, not in a way that reduces oughtness to isness (like how goodness is reduced to being).