We talked about two of Austin’s claims:
What I called Hart’s two small points apply to the first. They involve examples of laws that it is very difficult to construe as commands.
Hart’s example of the gunman is supposed to show that being threatened is not sufficient for having an obligation or duty.** Let’s treat “obligation” and “duty” as synonyms.
The example of speeding when there are no cops around is supposed to show that being threatened with a sanction is not a sufficient condition of having a duty.
John left us with an important question that we will revisit. When we’re talking about obligations or duties to obey the law, are we necessarily talking about moral obligations?
At one point, I said that you could express necessary conditions as sufficient conditions and vice versa. Alas, I forgot how to convert sufficient conditions.
First, let me be clear about what I am not saying. I am not saying that you can go from “A is a necessary condition of B” to “A is a sufficient condition of B”. No no no no.
Instead, what I’m saying is that if A is a necessary condition of B, then B is a sufficient condition of A. If A is necessary for B, then whenever you have B, you have A. You couldn’t have had B without its necessary condition, of course. But that means that B is a sufficient condition of A. Whenever you have B, you know that you will have A too.
By the same token, if C is a sufficient condition of D, then D is a necessary condition of C.†† This is the one I forgot in class. C’s being a sufficient condition of D means that C will always go with D. That means that you won’t find C without D or, in other words, that D is a necessary condition of C.
To use Liz’s example, if eating a big plate of spaghetti is sufficient to make Liz full, then Liz’s being full is a necessary condition of Liz’s eating too much spaghetti.
You have surely figured out the next thing I’m going to say. It is that necessary and sufficient conditions are not causes. Let me say it again. They aren’t causes. Hence, “being full” can be a necessary condition of “eating a big plate of spaghetti” even though you don’t feel full until after you have eaten the spaghetti.