A proof that ought implies can

Some actions are are things I can do immediately: for instance, I can immediately raise my hand. Others require that I do something to enable myself to do the action: for instance, to teach in person, I have to go to the classroom, or to feed my children, I need to obtain food. So, here is a very plausible axiom of deontic logic:

1. If I ought to do A, and A is not an action I can do immediately, then I ought to bring it about that I can immediately do A.

Now, say that I remotely can do an action provided that I can immediately do it, or I can immediately bring it about that I can immediately do it, or I can immediately bring it about that I can immediately bring it about that I can immediately do it, or ….

It follows from (1) and a bit of reasoning that:

2. If I ought to do A, then I remotely can do A, or I have an infinite regress of prerequisite obligations.

But:

3. It is false that I have an infinite regress of prerequisite obligations.

So:

4. If I ought to do A, then I remotely can do A.

Weak promises

Commanding is meant to create an obligating reason for another, while requesting is meant to create a non-obligating one. Promising is meant to create an obligating reason for self. There is a natural spot in illocutionary space, then, for a speech act meant to create a non-obligating reason for self, a speech act type that stands to promising as requesting does to commanding.

We would expect that when I have a normative power, I also have the corresponding weaker powers. If a legislature can bind under pain of ten years' imprisonment, they can bind under pain of a week's imprisonment. If I can create an obligating reason for myself, I can create a non-obligating reason for myself. That's another reason to think that we would have the "weak promise" speech act that creates non-obligating reasons.

I am not sure we have good phrases to express weak promises. We can approximate the force of a weak promise by weaselly promissory wordage like "I'll try to do this" or "I'll take your needs into account".