in which men may seek pardon for their sins. There must also be priests, from whom  we may receive penance for our sins, who may pray on our behalf and obtain for  us the aid of God and His providence. The public interest also requires that there  be magistrates appointed in the state, for through such persons, men pre-eminent  in the doing of justice, the law is given effect. For it is of little value that law  exists in the state if there are none to administer it.15
What private law is.
 16Private law is that which pertains primarily to the welfare of individuals and  secondarily to the res publica. Hence we say that it is in the public interest that no  one misuse his own. And so conversely, that which is primarily public looks secondarily  to the welfare of individuals. Private law has a threefold division: it is  deduced partly from the rules of natural law, partly from those of the jus gentium  and partly from those of the civil law.1718We have spoken above of public law and  private law. Now we must explain 19what natural law is, what the jus gentium is,  and what civil law (which may sometimes be called custom) is. For whom and  when, by custom or by a constitutio of the prince, the law may be curtailed is  discussed hereafter.20
What natural law is.
 21Natural law is defined in many ways. It may first be said to denote a certain  instinctive impulse arising out of animate nature by which individual living things  are led to act in certain ways. Hence it is thus defined: Natural law is that which  nature, that is, God himself, taught all living things. The word quod is then in  the accusative case and the word natura in the nominative. On the other hand,  it may be said that the word quod is in the nominative case, so that the definition  will be this: Natural law is that taught all living things by nature, that is, by  natural instinct. The word natura will then be in the ablative case.22 This is what  is meant when we say that our first instinctive impulses are not under our control,  but our second impulses are. That is why, if a matter proceeds only as far as simple  sensual pleasure, not beyond, only a venial sin is committed. But if it proceeds  farther, to the contriving of something, as where one puts into practice what he  has shamefully thought, it will then be called a third impulse and a mortal sin is  committed.23 And note that for the reason that justice is will, taking into account  rational beings only, natural law is impulse, regard being had to
15. D.184.108.40.206: infra 166, 305; Lewis in Speculum, xxxix, 262 n.