The Natural Law, as applied to the case of human beings, requires greater precision because of the fact that we have reason and free will. It is the our nature humans to act freely (i.e. to be provident for ourselves and others) by being inclined toward our proper acts and end. That is, we human beings must exercise our natural reason to discover what is best for us in order to acheive the end to which their nature inclines. Furhtermore, we must exercise our freedom, by choosing what reason determines to naturally suited to us, i.e. what is best for our nature.

Formally defined, the Natural Law is humans' participation in the Eternal Law, through reason and will. Humans actively participate in the eternal law of God (the governance of the world) by using reason in conformity with the Natural Law to discern what is good and evil.

One of the natural law theorist is Thomas Aquinas

natural inclination of humans to acheive their proper end through reason and free will is the natural law.

Natural Law Theory proposes that as physical laws of nature exist, so do universal moral laws. 

IMMANUEL KANT

Kant was an advocate of the natural law theory. An advocate of human dignity

(man as end in himself), supremacy of reason and free will (as God-given and inherent

in man), equality, freedom, and mutuality of rights, and universal law of morality.

It is reason that makes law and obeys law. Man knows what is natural right or

natural law because he is rational and the precepts of natural law are inherently written

in his heart and mind (conscience).

"Practical reason" (the "good will" in man; the "empirical imperative") that makes

law and compels the conscience of man to obey the law. Duty (to obey and revere the

law and to do good to fellow men) is the highest virtue.Doing an act not out of "duty"

(good will) is immoral, though it may be legal.His ideal society is one where all men

possess the virtue of duty to do good.

Man is a moral individual. He is not a chattel. Moral rightness is a matter of

"motives" and legal rightness refers to "external acts." In his Metaphysics of Ethics, Kant

wrote, "Every action is right which in itself, or in the maxim on which it proceeds, is such

that it can co-exist with the freedom of the will of each and all, according to a universal

law."

Kant criticized Rossaeu's social contract theory because Kant believed that

human rights are not contracted but are inherent in man (dignity, freedom, equality).

In a sense, Kant and St. Thomas agree that law is based on reason for the common good; that law is universal; and that natural law is inherent in the heart of men.

H. HEGELIAN PHILOSOPHY

Immanuel Kant influenced Hegel in his philosophy of law. Like Schelling and

Fichte, Hegel was an "speculative idealist" and an advocate of rationalism: "Whatever is

rational is real and whatever is real is rational." To him, reason is the ultimate essence

of the world or absolute reality.

Every concept leads to its opposite (thesis-anti thesis-synthesis) and that there is

an unending progress from thesis to antithesis and to synthesis, the latter being the

reconciliation of thesis and antithesis on a higher level. This is called the "dialectical

method" (the triadic process).

To Hegel, ethics culminates in the state and the state is the ethical idea and

reason turned into reality: "In the organization of the state, ... the divine enters into the

real." The state is a manifestation of the divine will. He wrote that all history is an

evolutionary process whose

ultimate goal is true liberty, and that liberty is only possible in a state, where man

reaches his dignity as an independent person.

He agreed with Roussaeu that in the "true state" it is the "universal" (the law) that

governs and "the individual of his own free will subjects himself to its rule." It is part of

the concept of man that he is free, Hegel wrote. The paradox was that in his latter

years, he opposed the democratic or republican form of government because of its

"subjectivism and atomism." He preferred the authoritarian state.

Hegel justified war or revolution based on and as an application of the dialectical method (struggle of ideas).

What Is Consistent with the Natural Law Is Right and What Is not in keeping with the Natural Law Is Wrong . 

Humans have a natural drive to eat, drink, sleep and procreate.  These actions are in accord with a natural law for species to survive and procreate.  Thus activities in conformity with such a law are morally good.  Activities that work against that law are morally wrong.  As an example consider that to eat too much or too little and place life in jeopardy is morally wrong.

1. Two Kinds of Natural Law Theory

At the outset, it is important to distinguish

two kinds of theory that go by the name of

natural law. The first is a theory of morality

that is roughly characterized by the

following theses. First, moral propositions

have what is sometimes called objective

standing in the sense that such propositions

are the bearers of objective truth-value; that

is, moral propositions can be objectively

true or false.

St. Thomas Aquinas, for example, identifies

the rational nature of human beings as that

which defines moral law: "the rule and

measure of human acts is the reason, which

is the first principle of human acts"

(Aquinas, ST I-II, Q.90, A.I). On this common

view, since human beings are by nature

rational beings, it is morally appropriate

that they should behave in a way that

conforms to their rational nature. Thus,

Aquinas derives the moral law from the

nature of human beings (thus, "natural

law").

For the theists there is a deity that created all of nature and created the laws as well and so obedience to those laws and the supplement

to those laws provided by the deity is the morally correct thing to do.

For atheists there is still the belief that humans have reasoning ability and with it the laws of nature are discernable.  For atheists who accept this approach to act in keeping with the laws of nature is the morally correct thing to do.