Moral certitude is a degree of certitude, based on currently available limited knowledge, sufficient to act in good conscience, i.e. without actual sin. Moral certitude is not absolute certitude. It is inherent to the fallen state that human persons have limited knowledge and a fallible use of reason when making judgments of the prudential order. A person need not have absolute certitude as to the best course of action before acting. The certitude possessed is morally sufficient if the individual makes use of available knowledge, and then chooses to act, or to refrain from acting, in conformity with the moral law, as it is understood by faith and reason. Use of moral certitude applies especially to the third font, since numerous different factors in the circumstances must be evaluated as to the totality of their moral weight.
Evaluating the consequences often involves a judgment about the degree of risk and the degree of possible harm. There may not be absolute certitude as to the exact set of consequences that will result from an act. However, there may be moral certitude that a certain degree of risk, and a certain degree of harm, is present. In such cases, the consequences in third font are evaluated based on the likelihood of possible harm and the extent of that harm, versus the likelihood of possible benefit and the extent of that benefit.
If there is a slight likelihood of great harm, or a great likelihood of slight harm, the bad consequences have only a slight moral weight. If there is a slight likelihood of great good, or a great likelihood of slight good, the good consequences have only a slight moral weight. Many daily activities carry some minor risk, such as crossing the street, or taking a short drive. But since the possibility of great harm is slight, the act need only offer a correspondingly minor benefit to be moral. However, the substantial possibility of grave harm would have substantial moral weight, even though the harm is only a risk and not a certainty.
Example: (1) A man decides to drive his vehicle while tired. His intention is good, in that he intends to get to his home. Driving a vehicle while tired is not intrinsically evil. Whether or not it is moral is a matter of degree, and this degree is in the circumstances. If he is extremely tired, so that there is a substantial risk of causing serious harm, then the act would be immoral. If he is not so tired as to present any significant risk of harm, then the act would be moral. So in the third font, there is a judgment to be made of the risk of bad consequences. This depends on how likely it is that his tiredness will result in an accident causing grave harm. If he is very tired, and the drive is long, the risk of grave harm in the consequences is greater. Even though the man cannot know in advance whether or not he will do harm with his vehicle, he is able to judge the degree of likelihood and the degree of possible harm, not with absolute certitude, but with moral certitude.
But when an act is immoral due to the great likelihood of great harm, even if the likely grave harm does not happen to occur, the morality of the act is unchanged. The morality of the act is based on the known likelihood of harm at the time that the decision to act was made, and is not based on the specific subsequent outcome. Since the person is only able to judge in advance, with moral certitude, the likelihood of the bad consequences, not the exact actual outcome, the morality is based on that likelihood (i.e. the risk of bad consequences).
Example: (2) A man decides to drive his vehicle, even though he knows that he is so tired there is a risk of serious harm (since he might fall asleep at the wheel, crash his vehicle, and cause injury or death). Even if he successfully drives the vehicle home without any harm, the act was still a sin, because the morality of the third font is based on what can be known, the known degree of risk, and not on the particular outcome in a particular case, which cannot be known in advance.
Similarly, when an act is moral due to the very small risk of great harm, even if the unlikely great harm happens to occur, the morality of the act is unchanged. Even though great harm resulted, the act was moral because the great harm was very unlikely, and so was outweighed by the likely good consequences.
Example: (3) A physician intends to cure a patient with a medication, which includes a small risk of grave harm (death), and where the benefits are substantial and very likely, and the risk is very small. If, by chance in a particular case, the patient dies, even though the likelihood of this outcome was very small, the act of giving the patient this medication was moral, because the physician had no way of knowing the outcome in this particular case. He correctly judged the known risk of serious harm to be small, and the benefits to be much more substantial than the risk, and so his decision to act was moral.
If physicians never treated patients, except when there was no risk at all of harm, there would be no medications, no treatments, and certainly no surgery, since nearly every medication and treatment has at least a very small risk of serious harm. There is a great benefit to society in general, and to particular persons, in accepting a limited amount of risk in exchange for more weighty benefits. Without this concept of acting despite bad consequences or the risk of bad consequences, very many good acts done with good intention could not be done, and society as well as individuals would suffer a very great loss from the inability to act despite limited bad consequences or a mere possibility of harm.
As long as the good circumstances are judged with moral certitude to outweigh the bad circumstances, the third font is moral. This is true even though the future consequences are not entirely knowable, and even when an unlikely bad consequence happens to occur. Judgment with moral certitude is based only on what is knowable. Whatever is truly and entirely unknowable does not affect the morality of an act.
Above text excerpted from my book: The Catechism of Catholic Ethics
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