1. At times doubts of fact lead to difficulties in applying rules of positive law, which have moral force only insofar as they specify the duty to obey law. The law itself sometimes provides for ways of settling such doubts; while at other times they can be obviated by a procedure of legal inquiry or by obtaining a dispensation from the law.
2. If a doubt of fact must be resolved without help from the law itself, one may resort to reasonable presumptions. This method is based on a general consideration of fairness: No one can be expected to do more to abide by the law than reasonable, law-abiding people do. Hence, one may resolve doubts of fact by any judgment of probability which such persons would accept.
3. Laws are not made to immobilize people but to guide common life. If doubts of fact are not readily resolved, however, law does tend to immobilize rather than help people cooperate fruitfully. Therefore, reasonable, law-abiding people who must resolve doubts of fact without the help of the law itself proceed on whatever judgment of fact seems probable.
4. Sometimes, however, a doubt of fact leads to difficulty in applying a specific moral norm.
5. In some cases, uncertainty about matters of fact leads to doubt about the gravity of an act recognized to be immoral. Perhaps one does something spiteful, in doubt about how much damage the act will do. Or one takes a pill to prevent birth, not knowing whether it works by abortion. When the doubtful fact bears on settling an issue of this sort, one should assume that the true state of affairs is that which renders the act more seriously wrong. For a person who purposely does what might destroy, damage, or impede a particular human good is willing actually to do so; and, morally speaking, anyone who is willing to do a wrong does that wrong. In general, a person who is willing to do what is wrong and is uncertain because of a doubt of fact how grave that wrong will be, is willing to do the more serious wrong.
6. In other cases where a doubt of fact leads to difficulty in applying a specific moral norm, the issue is whether any of the modes of responsibility will be violated if one acts in that state of doubt. In such cases, one can reach a judgment of conscience despite lack of certainty about the facts. The state of doubt itself is a matter of fact to be considered together with other relevant facts of the situation to which moral norms must be applied.
7. For instance, if one is not sure whether a side effect which one usually ought not to accept will occur, the doubt of fact should be resolved in the affirmative—expect the worst—if resolving it in the negative would be unfair, unfaithful, or against any other mode. Normally, for example, one should assume that drunken driving will lead to someone’s injury or death; almost never should one assume that the bad side effects will be avoided. The reason is that very seldom can one take such risks with the well-being of others without being grossly unfair to them.
8. Still, a level of doubt about the safety of driving which normally would indicate that one ought not to drive might be accepted without violating any of the modes—for example, if there is no other way to save someone’s life. In such a case, reasonable persons would wish others to take some unusual risk on their own behalf or on behalf of those they love, and so in fairness would approve acting with a certain level of doubt.