Many will claim that what we refer to as the law of nature or the moral law is merely our instincts and the Herd Instinct. We all know about instincts like feeling hungry, or motherly love, or sexual instinct. Sometimes we may feel the desire to help another person in agreement with the herd instinct; but feeling that desire and choosing it are two different things. What happens if you hear a cry for help from someone in danger; helping them will place you at risk. You will be conflicted with two desires; one to provide assistance and another to keep out of it due to the instinct for survival. Yet something inside of you tells you that you should follow the instinct to help and deny the urge to keep out. What is this? It is the moral law. You probably want to be safe more than you want to help the person in trouble, but yet something inside you causes you to follow the weaker instinct. It is the moral law.
Throughout history we have viewed some instincts as better than others. As in the example above we may say that the instinct to help someone in trouble is better than others. But instincts themselves are neither good or bad. Certainly instincts such as the sexual instinct or the instinct to fight must be restrained more often than an instinct such as motherly love. But a husband must follow his sexual instinct with his wife at certain times and a soldier must follow his instinct to fight at the right moment while the instinct of motherly love may have to be restrained in order to be fair to other children, and can even in and of itself be perverted. Even the mere idea that an instinct can be perverted tells us it can be good, pointing to good and bad, or right and wrong – a standard.
To choose the right instinct at the right time requires something other than the instincts themselves. It is much like the keys on the keyboard I am using to type this. The keys themselves do not determine which should be typed when. Something above them does. In order for what is typed to make words and sense something other than the keys must determine which key should be pressed and when.
In addition, a standard determines the quality of the output of that decision. We call some documents and stories good while others are not. Even simpler, for a word to be a word the letters must be typed in a certain order. As our instincts are used there is a standard we use to determine when they should be followed and when they should be restrained.  We do not merely follow them or restrain them, we choose to do one or the other. Our instincts cannot choose themselves which to follow. Something above them must make this choice – the moral law.
credit to C.S. Lewis – “Mere Christianity”