649. From the point of view of the arguments on which it is based, certitude is of two kinds. (a) Demonstrative certitude is the assent that rests on a conclusion logically drawn from certainly true premises. Example: Caius argues that he is obliged to go to Mass on Sunday, because the law is certain, and it is also certain that the law applies to him. (b) Probable certitude, which is the assent that rests on a conclusion, whose premises, though not certain, seem to be true, and against which there is no counter conclusion, or none that cannot be readily answered (see 703). Example: Caius is pretty sure that he is seriously ill, because he perceives a number of alarming symptoms; the possibility that these may be due to imagination is excluded by the fact that they are new and sudden. Caius, therefore, concludes that he may hold himself excused from attendance at Mass. 650. Probable certitude is sufficient for conscience, for in moral matters it is impossible to have at all times reasons that amount to a demonstration, and hence a person acts prudently in following a decision that is solidly probable and unopposed by any contrary serious probability. What is called “probable certitude” here is very different from probable opinion, about which there will be question below (662 sqq.) 651. From the point of view of the manner in which it is obtained, certitude is again twofold. (a) Direct certitude is that which is obtained from principles that are intrinsic to the case by applying to the matter the law concerning it. Examples: A judge who decides according to the evidence and proofs given in court that an accused is guilty, and a son who concludes from the Fourth Commandment that he is bound to help his parents in necessity, have direct certainty in their judgments, because they argued from principles that deal with the question before them. (b) Indirect certitude is that which is obtained from principles that are extrinsic to the case by applying to the matter in hand reflex principles (i.e., rules that direct how one should act in doubt) or the principle of authority (i.e., the argument drawn from the opinion of those who are acknowledged as competent to decide). Examples; If a judge is not able to form a certain judgment from intrinsic reasons concerning an accused, because strong arguments have been given both for guilt and for innocence, he has recourse to principles that have reference to his own state of doubt, and which declare that he must acquit when he is not certain of guilt. If a man is not able to decide whether the Fourth Commandment obliges him to keep his grandparents or mother-in-law in his home, when they upset his family and are able to take care of themselves, he can have recourse to the external principle of authority by consulting his confessor.