Some moral obligations can be partially fulfilled. For example, suppose that vegetarianism is morally required. If you eat meat only once a week, then you are partially fulfilling the moral obligation to be vegetation. Or suppose that you are morally required to give 10% of your income to charity. If you give 5%, then you are partially fulfilling the moral obligation to give 10%.

Now compare the following:

1. 100% vegetarianism
2. 99% vegetarianism
3. 98% vegetarianism

(1) is morally better than (2), which is morally better than (3). But is the moral difference between (1) and (2) greater than the moral difference difference between (2) and (3)? Is there something especially morally good about fully fulfilling a moral obligation? Is going from (2) to (1) morally better than going from (3) to (2)?

Another way of getting at the same point: (3) is morally worse than (2), which is morally worse than (1). But is there something especially morally bad about not fully fulfilling a moral obligation? Is going from (1) to (2) morally worse than going from (2) to (3)? If so, then there is diminishing marginal moral badness to meat eating: eating the first hamburger is morally worse than eating the second hamburger.

For vegetarianism, I’d lean towards saying that the moral difference between (1) and (2) equals the moral difference between (2) and (3). But we might want to say different things about different moral obligations. For example, consider the moral obligation to be faithful to your romantic partner by not cheating on him or her. Compare the following:

1. 100% faithfulness
2. 99% faithfulness
3. 98% faithfulness

In this case, it seems clear to me that going from (1) to (2) is morally worse than going from (2) to (3): the moral difference between cheating zero times and cheating one time is greater than the moral difference between cheating one time and cheating two times.

Assuming all this is right, what explains the difference between vegetarianism and faithfulness? One possible answer is that the marginal harm of faithlessness diminishes, but the marginal harm of meat eating does not diminish. But I’m not sure this explanation is right. One problem: if your cheating remains secret, then it’s arguably not harmful at all. But even in this case, it seems to me that going from 100% faithfulness to 99% faithfulness is morally worse than going from 99% faithfulness to 98% faithfulness.

Another possible answer is that the moral badness of faithlessness consists largely in the fact that it is a breach of trust. And 98% faithfulness is not a significantly further breach of trust than 99% faithfulness. In contrast, the moral badness of meat eating consists simply in the fact that it harms animals, not in any breach of trust. And going from 99% vegetarianism to 98% vegetarianism harms animals just as much as going from 100% vegetarianism to 99% vegetarianism does.

Complications

• It’s unclear how to work out the exact to which you’ve fulfilled a moral obligation. For example, is the extent to which you’ve fulfilled the moral obligation to be vegetarian proportional to the number of non-vegetarian meals you’ve eaten, the number of animals you’ve eaten, the pounds of meat you’ve eaten, or something else?
• I asked how “morally good” it is to fulfill a moral obligation to various extents. This can be understood in a couple different ways:
  • The praiseworthiness of fulfilling the moral obligation to a given extent
  • The strength of the moral reason to fulfill the moral obligation to a given extent
• Your actions can influence other people. Even if 100% fulfillment of a moral obligation is not in itself disproportionately morally good, it might set a good example. Conversely, anything less than 100% fulfillment might set a bad example. If this is right, then going from 99% vegetarianism to 100% vegetarianism might do more good than going from 98% vegetarianism to 99% vegetarianism, in virtue of the influence you have on others.