316. Verbal interpretation of the Natural Law is made as follows: (a) by private authority--that is, by those who are competent, on account of learning and prudence, to understand the meaning of the law, such as moral theologians; (b) by public authority--that is, by those who are appointed to rule, with the prerogative of declaring the meaning of the Natural Law. The Pope, since he must feed the flock of Christ, is divinely constituted to interpret Natural Law, and does so authentically and infallibly. Thus, the Church declares that certain matrimonial impediments are natural, and therefore incapable of being dispensed. On the competence of the Church to give authoritative interpretations of the natural law in the field of morals, Pius XII has spoken clearly and forcefully: “... it must openly and firmly be held that the power of the Church has never been limited to the boundaries of strictly religious matters’ as they are called; but the whole content of the natural law, its institution, interpretation and application are within its power insofar as its moral element is concerned. For the observation of the natural law, by the ordination of God, is the way by which man must strive to attain his supernatural end. On the road to this supernatural end. it is the Church that is his leader and guide. This is the way the Apostles acted, and from the earliest times the Church held to this way of acting as it does today--and not in the manner of a private leader and counselor, but from the command and authority of God” (AAS 46 [1954] 671-672). 317. From the foregoing it follows that the Natural Law is so unchangeable that it cannot be abrogated or properly dispensed, or given an emendatory interpretation. But, though the law itself remains, there are cases in which non-observance of it is excused from guilt. These cases can be reduced to physical and moral impossibility. (a) In cases of physical impossibility (i.e., when the powers requisite for observance are wanting), one is manifestly excused; for law is reasonable, and it is not reasonable to require impossibilities. Examples: Infants are not guilty of sin against the Natural Law, when they do not pray; for they lack the use of reason, which is presupposed by the notion of prayer. He who is unable to work is not obliged to earn support for relatives. (b) In cases of moral impossibility (i.e., when a law cannot be kept without the infringement of a higher law or the loss of a higher good), one is also excused; for it is unreasonable to prefer the less to the more important. Example: Titus lends a revolver to Balbus. Later he asks that it be returned to him, as he wishes to kill himself. Now, property is less valuable than life, and hence Balbus is unable in this case to observe the law which requires that things borrowed must be returned.