486. Authentic interpretations of ecclesiastical laws are given by the legislator, his successor, or one delegated by either (Canon 17, Sec. 1). (a) The Pope is the authentic interpreter of all ecclesiastical laws. A special commission appointed by the Pope interprets the general law of the Code. (b) The bishop is the authentic interpreter of diocesan laws made by himself or by his predecessors. 487. Cessation of Obligation.--The ordinary ways in which a law ceases to be obligatory for an individual are: (a) on the part of the subject, that he ceases to be subject to the law (exemption), or is unable to observe it (excuse); (b) on the part of the lawgiver, that he removes the obligation for the individual (dispensation). 488. As to exemption from Church laws note: (a) he who ceases to be subject to the law (e.g., one who has received a privilege of exemption, or who has departed from the place where the law is in force), is of course not obliged by the law; (b) neither is he guilty of any fault if he brought about his freedom only just before the law became effective and with the sole purpose of being exempt; for the law does not oblige that one remain subject to it. 489. Excuses from the law are reduced to two, namely, ignorance and impossibility. (a) Ignorance excuses from the guilt of non-observance, if it is inculpable (see 24 sqq.). The question now is whether or not and when it excuses from legal consequences, such as invalidity, penalty, reservation of sin, etc. (b) Impossibility excuses from both obligation and guilt. 490. Ignorance of ecclesiastical law or of a penalty attached to the law has the following effects determined in the law: (a) No kind of ignorance excuses from irritating or inhabilitating laws, unless the contrary is expressly provided for in the law itself (Canon 16, Sec. 1). Thus a person who contracts marriage, while ignorant that he and the other person are first cousins, is invalidly married. (b) Affected ignorance of ecclesiastical law or of the penalty alone does not excuse from any penalties _latae sententiae_ (Canon 2229, Sec.1). (c) If the law contains the following words: _praesumpserit, ausus fuerit, scienter, studiose, temerarie, consulto egerit_, or others similar to them which require full knowledge and deliberation, any diminution of imputability on the part of either the intellect or the will exempts the delinquent from penalties _latae sententiae_ (Canon 2229, Sec.2). (d) If the law does not contain such words, crass or supine ignorance of the law or even of only the penalty does not exempt from any penalty _latae sententiae_; ignorance that is not crass or supine exempts from medicinal penalties, but not from vindicative penalties _latae sententiae_ (Canon 2229, Sec.3, 1).