What passages of philosophy are worth thinking about today, January 20th, the day that Donald Trump is to be sworn-in as president of the United States of America? I imagine that there will be a diversity of suggestions.

Here’s mine, from the recently deceased Derek Parfit’s Reasons and Persons. It appears in the section “Five Mistakes in Moral Mathematics,” and it’s about how to think about individual actions that have very small or even imperceptible positive effects, and so may seem not worth it.

Some people believe

(The Fifth Mistake) If some act has effects on other people that are imperceptible, this act cannot be morally wrong because it has these effects. An act cannot be wrong because of its effects on other people, if none of these people could ever notice any difference. Similarly, if some act would have imperceptible effects on other people, these effects cannot make this act what someone ought to do.

One kind of imperceptible effect is not controversial. I may cause you serious harm in a way that is imperceptible. The dose of radiation that I give you may be the unknown cause of the cancer that kills you many years later. Though the cause may be unknown, the effect is here perceptible. But, in the cases I shall be considering, the effects are imperceptible.

Consider first a variant of a case described by Glover.

The Drops of Water. A large number of wounded men lie out in the desert, suffering from intense thirst. We are an equally large number of altruists, each of whom has a pint of water. We could pour these pints into a water-cart. This would be driven into the desert, and our water would be shared equally between all these many wounded men. By adding his pint, each of us would enable each wounded man to drink slightly more water—perhaps only an extra drop. Even to a very thirsty man, each of these extra drops would be a very small benefit. The effect on each man might even be imperceptible.

Assume that the benefit given to each man would be merely the relieving of his intensely painful thirst. There would be no effect on these men’s health. Since the benefits would be merely the relief of suffering, these are the kind of benefit of which it can most plausibly be claimed that, to be benefits at all, they must be perceptible.

Suppose first that, because the numbers are not very large, the benefit that each of us would give to each man would, though very small, be perceptible. If we make the Fourth Mistake, we believe that such tiny benefits have no moral significance. We believe that, if some act would give to others such tiny benefits, this cannot make this act what someone ought to do. We are forced to conclude that none of us ought to add his pint. This is clearly the wrong conclusion.

Assume, next, that there are a thousand wounded men, and a thousand altruists. If we pour our pints into the watercart, each of us will cause each wounded man to drink an extra thousandth of a pint. These men might notice the difference between drinking no water and one thousandth of a pint. Let us therefore ask, ‘If these men will drink at least one tenth of a pint, could they notice the effect of drinking any extra thousandth of a pint?’ I shall assume that the answer is No. (If the answer is Yes, we merely need to suppose that there are more altruists and wounded men. There must be some fraction of a pint whose effect would be too small to be perceptible.)

Suppose that a hundred altruists have already poured their water into the cart. Each of the wounded men will drink at least one tenth of a pint. We are the other nine hundred altruists, each of whom could add his pint. Suppose next that we make the Fifth Mistake. We believe that, if some act would have imperceptible effects on other people, these effects cannot make this act what someone ought to do. If we believe this, we cannot explain why each of us ought to add his pint. It may be said: ‘We can avoid this problem if we redescribe the effect of adding each pint. We need not claim that this gives to each of the men one thousandth of a pint. We could claim that it gives to one man one pint.’

This claim is false. The water will be shared equally between all these men. When I add my pint, is the effect that an extra man receives a full pint? If I had not added my pint, is there some man who would have received nothing rather than a full pint? Neither of these is true. There is only one correct description of the effect of my act. It gives to each of the thousand men an extra thousandth of a pint…

What we can appeal to is a claim about what we together do. We can claim

(C10) When (1) the best outcome would be the one in which people are benefited most, and (2) each of the members of some group could act in a certain way, and (3) they would benefit these other people if enough of them acted in this way, and (4) they would benefit these people most if they all acted in this way, and (5) each of them both knows these facts and believes that enough of them will act in this way, then (6) each of them ought to act in this way.

Each of us could give to each of the thousand wounded men an extra thousandth of a pint of water. If enough of us act in this way this will benefit each of these men. And we will benefit these men most if we all act in this way. We know these facts, and we know that enough of us—one hundred—have already acted in this way. (C10) implies correctly that each of us ought to act in this way.

Remember now the Fifth Mistake. On this view, an act cannot be right or wrong, because of its effects on other people, if these effects are imperceptible. The case just described refutes this view. It is clear that, in this case, each of us should pour his pint into the water-cart. Each of us should cause each wounded man to drink an extra thousandth of a pint. Each of us ought to affect each wounded man in this way, even though these effects are imperceptible. We may believe that, because these effects are imperceptible, each of us is benefiting no one. But, even if each benefits no one, we together greatly benefit these wounded men. The effects of all our acts are perceptible. We greatly relieve the intense thirst of these men.

(Derek Parfit, Reasons and Persons, pp. 75-77)