I will admit, I don't like talking about "Natural Law" much - I don't like talking about how nature itself shows us how things are supposed to be. Not because this isn't true, but for two reasons.
1. It's Law, and the Law doesn't save anyone.
2. Sinful Man will burden one another by their flights of reason and imagination "based" on Natural Law.
First, as I see it, more and more focus is given to natural law as an apologetic tool - the way that we can try to convince the heathen world out there that they need to do what we as Christians know to be right and true - it's "obvious" from nature. Rule number 7 of rhetoric... any time you write "obviously" or "its obvious that", it's neither obvious nor agreed upon.
What we forget is this - to sin, to live sin is to fundamentally go against nature as God designed us - to sin is to not be human, to be fallen. So if one is trapped in sin, one isn't going to care about natural law (unless it's a philosophic interest, but even then ones own pet sins will be ignored). Natural Law does nothing to convince or change minds - really it doesn't. It isn't arguing in the left hand kingdom... Natural Law points to sin, and sin is always ignored in the world. The world cares about benefits and how something impacts me - God's Law (natural or otherwise) is always concerned about the neighbor... so the world won't recognize it.
Second, and this is the bigger one for me (if people want to engage in apologetics, let them) - we take natural Law and run with it in ways it is not meant to go. We will let that old harlot, dame reason, jump on in, and then we will build new constructions and laws on top of "natural" Law, things that should be evident and flow... and we just give burdens to others.
Allow me an example that is absurd. Let's talk about eating. It should be obvious from nature, that food was given to man for two main reasons:
1. To provide nourishment.
2. To provide enjoyment.
Therefore, we can say that a Christian ought never eat Cotton Candy, for it does not really provide nourishment, but rather is just a matter of enjoyment. More over, it is a totally artificial food, overly processed beyond compare. It is a foul, abnormal abomination. Cotton Candy is evil.
This flows quite well logically. Really, it does. Try to tell me that cotton candy provides actual nourishment or that it is health, or that it is natural. Therefore, it violates natural law, and you can't eat it.
(Oh, but no one says something like that, it's silly. Well, I've been told that a Christian needs to avoid eating M&Ms... more than a handful is a sin. And besides, I'm looking at patterns of thoughts here)
Here's the problem - does God forbid eating food for pleasure? No. He forbids gluttony - but the kid at the fair having a wad of cotton candy isn't necessarily gluttony. But, if we put such an emphasis upon what we think are the natural, the obvious purposes of something in creation, it gives us license to derive all sorts of "laws" that are not part of God's command. It gives us license to be new law-givers.
And we take this and we make burdens with it, all for the best intentions. "We must say that Cotton Candy is evil. Don't we see the culture of obesity that we have today, what a scourge this is. Cotton Candy is part of that culture, and we must speak out against it!"
And we are given over to our own pet crusades, and we place burdens upon one another that God did not place upon us in His Word.
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Actually, what it boils down to is this. We have enough to ponder in God's revealed Word. Why are we spending so much time in the realm of "nature" and "reason", when there is Scripture enough to live and rely upon? Why are we so worried about trying to make others be good... as though our works make others to be more righteous? This ends up becoming idolatry and self-worship. Let it be avoided.