I use the following example to explain the concepts of necessary and sufficient condition to my students:
Shelbie (my oldest dog) gets a treat if and only if Shelbie is good.
This says two things:
- Shelbie gets a treat if Shelbie is good; and
- Shelbie gets a treat only if Shelbie is good.
The first says that Shelbie's being good is a sufficient condition of Shelbie's getting a treat. Nothing else, in other words, is necessary. Being good is enough; it's all it takes.
The second says that Shelbie's being good is a necessary condition of Shelbie's getting a treat. If she's not good, then she doesn't get a treat.
What would falsify the statements? The first statement would be falsified if Shelbie were good but did not get a treat. Shelbie herself, if she could speak, would protest, "Hey! You said I'd get a treat if I were good. I was good, so where's my treat?" The second statement would be falsified if Shelbie were not good but still got a treat. Shelbie would not likely complain about this, since she likes treats; but if her only concern were the truth, she would protest that I had said something false.
Normative ethical theories set forth necessary and sufficient conditions for moral rightness. Act utilitarianism, for example, says that an act is right if and only if it maximizes net utility. This says that maximization of net utility is necessary and sufficient for rightness. There are two ways to falsify this theory: (1) by describing an act that is right (or seems right, intuitively) but does not maximize net utility; and (2) by describing an act that maximizes net utility but is not right (intuitively).
There are many normative ethical theories. I teach 12 of them in my Ethics course. Each theory states a different necessary and sufficient condition for moral rightness. I think of theories as sorting devices. Each theory sorts acts into two categories: right and not right. ("Not right" means "wrong.") Which theory one accepts or endorses depends on how well the alternative theories sort acts. If act utilitarianism does a better job of sorting acts than any alternative theory, as far as my intuitions are concerned, I will accept or endorse act utilitarianism. Different people accept or endorse different theories because different people have different intuitions. An act that seems right to me may not seem right to you, and vice versa.
