Each of us is a member of our mortal world and are therefore subject to its laws of deterioration, entropy, and death. Or in other words, the laws of physics are inescapable to us. We are forever under the powers of gravity, force, action and reaction, magnetism, electricity, temperature, and everything else that is baked into the matter of this world.
We do not have to like these laws, but we do have to adhere to them, simply because we have no other choice. These laws are not elected, they just are.
These laws are impersonal and unbiased. They can work for our bad, such as tripping and being pulled by gravity into a hurtful fall. They can work for our good, such as jumping in the air and being pulled back to where its safe instead of floating out of the atmosphere.
That is the nature of pure law. It is entirely unbiased. It enacts itself the same way to a king as to a slave, and it never varies in its order. Also, it never ceases to apply. At all times and in all places the laws are in full effect. In fact, multiple separate laws may apply to the same subject at the same moment, but each will have its full realization, none will be denied effect by another. They will each control what they control, and not what they don’t.
The laws of nature not only give our world structure and predictability, they also serve as excellent schoolmasters for understanding law itself. If there is a moral law, then to be a law it must also be impersonal and unbiased, just like natural law. There can be several facets of moral law, but to be a law, then each of those facets must have full expression, with no variance in how they execute themselves on one subject or another. To be a law, moral law must apply at all times and in all places, it simply must be. Also, to be a law, it must apply to what it applies, and not to what it does not.
And that last point is why we find it far more difficult to accept the existence of moral law than natural law. For where natural law applies to the forces of nature exclusively, moral law applies to the soul exclusively. Thus we can see, hear, and touch the evidence of natural law with our external senses, but we cannot perceive the effects of moral law with our external senses. That simply is not the realm of its jurisdiction. We only perceive them in our heart. Not only that, but where the forces of natural law are often immediate, the effects of moral law are often enacted over a prolonged amount of time, making it difficult to draw the correlation of cause and effect.
But though it is harder to recognize moral law, it is still there.