Natural Law

[Tantum Ergo]

So, who thinks they have a good understanding of natural law, and how would you explain the basic premise to others? I find over and over again that Catholics try to make arguments from natural law (because they have been told they should do so, rather than make an argument from Catholic teaching itself), and they always seem to explain it poorly. Many Catholics seem to think what is simply biologically natural equals natural law - for example, that NFP is ok and contraception isn't because NFP is natural and contraception is artificial. And yes, that's somewhat true, but it's not all the way there. Dying your hair is also artificially interfering with your body but it's not immoral. 

 

Since few secular people are educated on natural law, a Catholic trying to explain their beliefs from that system again ends up having to explain something the secular person doesn't understand. 

 

Does anyone know any good resources that clearly explain natural law, especially one that would be appropriate for group study? (Like a Bible study). 

 

Sorry if this doesn't belong in debates. I wasn't quite sure where to put it.