A proof for the existance of God is that the source of natural law can only come from something above us and that natural law must be the voice of God within the soul. My question is why can’t the source of natural law be something material rather than immaterial? I’m not talking about something like an instinctive desire but something that has been inherited in each one of us to give a sense of obligation to do good rather than evil.
Your question is actually much closer to the reality of what Natural Law is. We can only speak of “the voice of God within the soul” at a theological level, not a strictly philosophical one, and certainly the first proponents of natural law (Aristotle and Cicero) did not believe in God as we think of him.
Natural Law is perfectly compatible with Catholic teaching, and why St. Thomas was able to “flesh it out” with a deeper theological understanding, but it is not primarily a religious view, and certainly not immediately a proof for God.
Natural Law, as you correctly guess, IS derived from our experiece of the natural, material world. We see that everything that exists seems to have a purpose, (what Aristotle called an “end” or “finality”), a natural trajectory of development that is tied up with what the thing is. Thus, because a tree is a certain way, it fulfills that way of being by growing from a seed into a full-fledged oak, for example.
With humans, however, we can discover that a far “deeper” purpose seems to be at stake. Our fulfillment is not merely tied up in being a complete physical human being (like a full-grown tree), reproducing, surviving and taking in food, but we crave a spiritual, non-material happiness. Unlike trees and animals, we are intelligent, rational creatures; this is what makes us human, and so it is only by seeking to know, and contemplating Truth that we can be truly happy.
Natural Law derives its ethical structure from this finality that we have as humans. Right and wrong are measured against this “end,” and thus killing, cheating, adultery, etc. etc. (all the sins you can think of) are wrong because they go against our purpose and our nature. They lead us away from being full, complete humans. Becasue we are rational creatures, we can discern what is right and wrong based on this, and make the right choices.
To speak of a conscience placed in us by God is something we can say theologically, but not philosophically, even though the two are compatible. We know as Christians that God is the source of Natural Law, and he is also the source of all theological revelation and the source of our intelligence/rationality.
I don’t know if this makes sense, or even starts to answer your question. But you have to be careful in talking about Natural Law as “the voice of God” because Natural Law is a philosphical system, which uses reason alone, and not faith or religious beliefs. That is why the ethics we can discern with Natural law are the same for all people, everywhere, regardless of their religious beliefs or culture; they are discovered with human reason alone based on our experience.