One of the strategies employed in apologetics is the attempt to work from premises that the opponent holds. This makes sense from a tactical standpoint. If the opponent doesn't believe that the Bible is not the inspired Word of God, appealing to the Bible's authority is a moot point.
Sometimes however in an attempt to do this Catholic apologists commit a different fallacy. Oftentimes with atheists we attempt to argue about issues using Natural Law. One can see this with the debate surrounding marriage.
One of the fallacies I've noticed (and am guilty of myself) is the attempt to employ Natural Law without the existence of God as a premise. This I have concluded to be faulty, and causes unnecessary confusion.
It makes sense at first. One can note the directed nature of entities in existence (i.e. final causes) without necessarily noting that such "directedness" only makes sense with a "Director". Thus since atheists reject the notion of God one can at first appear to argue Natural Law without invoking God.
For the record, this is not "God" in the sense of how the Catholic Church views Him (though there are a lot of similarities.) This is the God of the Greeks, or the God of "classical theism." The God whose attributes are discovered through human reason.
There is however one huge problem with ignoring the existence of God. It is worthwhile to note the intended nature of things given their directed nature. But without God the question remains, "Assuming such things as final causes exist, why conform to them?" It is the same question we pose to atheists, "Why be good?"
Natural Law at its heart is the study of entities and their directed nature. Natural Law states that entities are directed to an end. That end is intended by that which directed it, i.e. God. Since God is pure Good, the natural end of entities are in fact good. And it is good to work toward those natural ends, and evil to frustrate them.
Without God the whole reason for acting toward the intended end of an entity goes out the window. The fact that an entity might be oriented toward a particular end is nice and all. But God is what makes the system moral. Otherwise we have entities of a directed nature that while interesting in principle have no justification for following them.
Both the material atheist and the Natural Law philosopher say that morality can be determined by observation and reason. But without God the atheist is in a pickle as to why this morality should be followed beyond self-centered reasons. When we as apologists try to exclude God from the conversation we run into the same problem.
Clearly a better understanding of Natural Law and how it relates to classical Theism as a whole is necessary. The Catholic laity is undergoing a revival of these long neglected yet timely principles. But we'd best be careful not to cherry pick. Like the doctrines of the Faith, Natural Law philosophy has a lot of pieces and they are there for a reason.
