I know that many deontic logicians would consider the following argument to be valid:
- If you’re going to behead Jones, then you ought to behead him using the sharp sword.
- You ought to behead Jones.
- Therefore, you ought to behead Jones using the sharp sword.
I know that it's controversial just what exactly the argument's form is. So let's just keep the form in ordinary English. Thus, I take the form to be:
For any subject, S, and for any actions, A and B,…
- If S does A, then S ought to do B.
- S ought to do A.
- Therefore, S ought to do B.
This is called deontic detachment (in contrast to the invalid form of factual detachment, where (2) would be 'S does A'). But it seems to me that deontic detachment doesn't just work where A and B range over only actions but also where A and B range over anything for which oughts apply (including intentions, beliefs, and desires). For instance, consider the following argument:
- If you believe that you wouldn’t quit smoking even if you were to promise your wife that you would, then you ought to refrain from promising your wife that you’ll quit smoking.
- You ought to believe that you wouldn’t quit smoking even if you were to promise your wife that you would.
- Therefore, you ought to refrain from promising your wife that you’ll quit.
This seems valid to me. And the form of this argument is as follows:
For any subject, S, and for any A and B for which oughts apply…
- If S As, then S ought to B.
- S ought to A.
- Therefore, S ought to B.
So I have two questions. First, can anyone think of any counterexamples to the form directly above? Second, does anyone know of any literature in which it is claimed that deontic detachment works not only for instances ranging over actions but also for instances ranging over combinations of things for which oughts apply?
Any suggestions about what to read as well as any suggestions about whether I should continue to consider such an argument form to be valid would be greatly appreciated.