CCC I. The Natural Moral Law

1954 Man participates in the wisdom and goodness of the Creator who gives him mastery over his acts and the ability to govern himself with a view to the true and the good.

The natural law expresses the original moral sense which enables man to discern by reason the good and the evil, the truth and the lie:

The natural law is written and engraved in the soul of each and every man, because it is human reason ordaining him to do good and forbidding him to sin . . . But this command of human reason would not have the force of law if it were not the voice and interpreter of a higher reason to which our spirit and our freedom must be submitted.5

1955 The "divine and natural" law6 shows man the way to follow so as to practice the good and attain his end. the natural law states the first and essential precepts which govern the moral life. It hinges upon the desire for God and submission to him, who is the source and judge of all that is good, as well as upon the sense that the other is one's equal. Its principal precepts are expressed in the Decalogue. This law is called "natural," not in reference to the nature of irrational beings, but because reason which decrees it properly belongs to human nature:

Where then are these rules written, if not in the book of that light we call the truth? In it is written every just law; from it the law passes into the heart of the man who does justice, not that it migrates into it, but that it places its imprint on it, like a seal on a ring that passes onto wax, without leaving the ring.7

The natural law is nothing other than the light of understanding placed in us by God; through it we know what we must do and what we must avoid. God has given this light or law at the creation.8

1956 The natural law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men. It expresses the dignity of the person and determines the basis for his fundamental rights and duties:

For there is a true law: right reason. It is in conformity with nature, is diffused among all men, and is immutable and eternal; its orders summon to duty; its prohibitions turn away from offense .... To replace it with a contrary law is a sacrilege; failure to apply even one of its provisions is forbidden; no one can abrogate it entirely.9

Catechism of the Catholic Church, I. The Natural Moral Law
http://www.vatican.va/archive/ENG0015/__P6U.HTM

Natural Law and Natural Rights

Treatise on the Laws [51 BC]
Author: Marcus Tullius Cicero (106-43 B.C.)

"For he who knows himself must be conscious that he is inspired by a divine principle. He will look upon his rational part as a resemblance to some divinity consecrated within him, and will always be careful that his sentiments, as well as his external behaviour, be worthy of this inestimable gift of God. A serious and thorough examination of all his powers, will inform him what signal advantages he has received from nature, and with what infinite help he is furnished for the attainment of wisdom. For, from his first entrance into the world, he has, as it were, the intelligible principles of things delineated on his mind, by the enlightening assistance of which, and the guidance of wisdom, he may become a good, and, consequently, a happy man." (par. 293)

—If then in the majority of nations, many pernicious and mischievous enactments are made, as far removed from the law of justice we have defined as the mutual engagements of robbers, are we bound to call them laws? For as we cannot call the recipes of ignorant empirics, who give poisons instead of medicines, the prescriptions of a physician, we cannot call that the true law of the people, whatever be its name, if it enjoins what is injurious, let the people receive it as they will. For law is the just distinction between right and wrong, conformable to nature, the original and principal regulator of all things, by which the laws of men should be measured, whether they punish the guilty or protect the innocent. (par. 374)

“There is indeed (says he) a law agreeable to nature, and no other than right reason, made known to all men, constant and perpetual; which calls us to duty by commands, and deters us from sin by threats; and whose commands and threats are neither of them in vain to the good, though they may seem of little force to the wicked. This law we are neither allowed to disannual, nor to diminish; nor is it possible it should be totally reversed; the senate or the people cannot free us from its authority. Nor do we need any other explainer or interpreter of it besides ourselves. Nor will it be different at Rome and at Athens, now and hereafter; but will eternally and unchangeably affect all persons in all places: God himself appearing the universal master, the universal king. It is he who is the inventor, the expounder, the enactor of this law; which whosoever shall refuse to obey, shall fly and loath his own person, and renounce his title to humanity; and shall thus undergo the severest penalties, though he escape everything else which falls under our common name and notion of punishment?” (par. 795)

Author: Marcus Tullius Cicero (106-43 B.C.)
http://oll.libertyfund.org/index.php?option=com_staticxt&staticfile=show.php%3Ftitle=545&layout=html