Ought Implies Can

The May issue of The Freeman is now available online here (full issue PDF).  The revised and extended version of my Ought/Can paper is in this issue and can be found separately here (HTML version). 

Comments

[Lee Kelly]

But there is an ought-can problem, so to speak. A can cannot be derived from an ought for the same reason that an ought cannot be derived from an is. Of course, just because you cannot derive a can from an ought does not mean you ought not to.

Most sensible people presuppose that what action ought to be taken must also be possible, but this is an additonal premise. Without such a premise to bridge the gap, deriving an can from an ought is like trying to derive Q from P.

[Lee Kelly]

Clarification.

Steve ought to fly to the moon
therefore,
Steve can fly to the moon

Formalised, this argument is:

P
therefore,
Q

That's why you can't derive a can from an ought (and its the same reason that you cannot derive an ought from an is). For the derivation to be valid we introduce another assumption, that is, if Steve ought to do something, then Steve can do it. Then we can get.

If P then Q
P
therefore,
Q

But, of course, someone could disagree with our assumption. They'd be stupid for it, but people can be stupid, even if they ought not to be. Some people might be tempted to say that no such additional assumption is needed since a can is implicit in an ought. But logically this makes no difference, it just obscures the assumption instead of making it explicit.

[Zac]

@Lee, yes they would be stupid for disagreeing with the assumption. Disagreeing with the assumption will lead you to do a lot of stupid things (If you believe you ought to do something to achieve an outcome, you will take those actions, but if it turns out that you cannot, that is, the actions will never achieve that outcome, you will be constantly disappointed).

Steve's essay is not a proof in formal logic of "if ought, then can," it is an explanation that if you do not accept "if ought, then can" as an assumption, you are unlikely to achieve your desired ends. He suggests (rightly) that the critics of economics are focused on what ends ought to be achieved but ignore what can be done to achieve those ends.

I will put the "ought implies can" argument another way. Consider the set of all outcomes O and its subset, all possible outcomes P. If you are a moral philosopher trying to determine which outcomes we ought to pursue, should you be selecting from O or P?

[Lee Kelly]

Zac,

Since I do not feel like playing Devil's Advocate, you'll get no argument from me. Steve's argument is a great argument, I think. I was just writing about the logic of the situation. You gotta be careful when you say something like 'ought implies can'.

[Arare Litus]

"advocatus diaboli" sounds so much more evil...

[LVDH]

Incidentally Roger Garrison´s paper in this issue of The Freeman is great too!