Aquinas points out that while the first principle of natural law, "good is to be done and pursued and evil to be avoided", is most certain, the more concrete and particular the principles and situations at which one looks, the less great is the certainty that can be attained. Consequently, anyone who sets out to develop or expound a moral system that will enable him to objectively judge every moral case with certainty, is sure to fail in this mistake: either there will be cases that cannot be judged with clarity by the system, or there will be cases that are misjudged by it. The following series of situations may serve as an illustration of inevitability of "grey" cases.
1. A mother is stranded with her child, aged 10, on an island, has no way to contact anyone off the island, and no way to procure additional fresh water. She knows that people will be arriving by boat in four weeks, as the island is a scheduled and never missed stop on a cruise. There is enough water for her child to survive four weeks, though getting somewhat dehydrated. She refrains from drinking any of the water, so that she can give it all to her child, though she knows she is morally certainty to die herself.
2. Ten adults are stranded with ten children in a similar situation. The adults all go without water for the sake of the children, and die of thirst.
3. Twenty adults are stranded on the island. Ten volunteer to give their share of the water to the others.
4. Twenty adults are stranded on the island, with adequate fresh water for all of them for 18 days. Thus, if they all drink a normal share of the water, they may well not all die, but will all run a serious risk of death. Five of them therefore volunteer to give up their share of water, and are thus virtually certain to die.
5. The same situation as four, with the additional remark that those who volunteer to give up their water persons with incurable cancer who expect to die within a few years, and so consider a greater chance of survival for the healthy and younger persons worth giving up their own chance of survival for.
6. The same situation as five, except that there is adequate fresh water for everyone for a little more than 20 days, so that if everyone drinks their share of water, the risk of death for each individual is relatively low, around 0.5%.
7. The same situation as six, except that the persons who give up their water are persons suffering from a disease that will kill them within two months in any event, and causes them constant and great pain, and this horrible pain and short life span plays a significant role in their decision to volunteer to give up their water.
It is quite clear, in the first three cases, that the persons who give up their water are not guilty of suicide either directly or indirectly, that is, either by reason of their intention (they will to die), or by reason of the objective character of their choice. (By "objective character" I mean that theoretically, it might be that giving up the water only makes sense if someone considers his life of no value, and is basically throwing it away; but this is clearly not the case in the first three examples.)
In the last case, if the risk of death given that everyone drinks water is low enough, and if the painfulness of the people's lives plays a great enough role in their deliberation, it is clear that their decision to give up drinking water is suicidal in its very intention. Similarly the sixth case is clearly suicidal in fact, if not in intention, and is morally wrong.
There is, however, a continuum between the first cases and the last cases–a continuum both in regard to the greatness of the risk of other people's death avoided by giving up one's own water, and in regard to the shortness of life and greatness of pain that makes one less concerned to preserve one's own life.
Consequently, it is impossible to make a system that will enable one to objectively judge every such decision as objectively right or objectively wrong. One might say that "if a person in giving up their water is objectively treating their life as though it were of no value, the decision to give up their water is wrong," but there will be cases in which it is unclear whether giving up their water is "treating their life as if it were of no value."