Thursday, April 12, 2007


Well conversed?

I was struck by this sentence in the SEP entry on Mill:

'This principle that must implies ought is the converse of the well-known Kantian principle that ought implies can.'

My first thought was 'No it isn't!'. Clearly it's the converse of ought implies must, which is pretty preposterous on the relevant reading: ought implies cannot but; ought implies unable not to (i.e. you ought to help the needy only if you cannot but help the needy, or you ought to help the needy only if you are unable not to help the needy). Not even Kant would want his fingerprints on this train-wreck of a principle.

Is there a charitable interpretation of the claim in the text that I'm missing?


I thought at first this was the contrapositive, but then realized that the contrapositive of "ought implies can" is "must implies may".

I assume they just mean "related conditional in the opposite direction" by "converse". It sounded natural enough to me at first, but I really can't think of other instances of this sort of usage.
I caught myself almost doing this in a paper as well. I think Kenny's diagnosis nailed it. There needs to be a name for this sort of thing. Maybe psuedo-converse? The pseudo-converse of p->q is q*->p, where q* is conceptually similar to q. That is vague enough to upset the logicians while still sounding sort of informative.
I think Kenny's right too. But it seems to me you can only pull this off when i. people can be confident you really do know the more proper sense of the term, and much more importantly ii. it's in a setting where such loose-usage isn't likely to be misleading given the intended audience.

I'm not in a position to comment on (i) in this case, but in an SEP entry on Mill, it looks like a clear violation of (ii). (Think of all the poor kids trying to find something accurate to plagarise..........)
"ought implies must"

I like it! I'm sure that some determinist has assented to this in an inspired moment of bullet-biting.
I agree that the wording is ill-chosen, but I think the author has something like this in mind:

Can X = X is possible / Agent is capable of doing X.

Must X = X is necessary / Agent is required to do X.

Ought = some ill-defined deontic modal in between, such that

If agent A ought to X, then A can X.

(Where the **real** converse fails).


If agent A must X, then agent A ought to X.

(Where the **real** converse fails).

Thus, I think that, for this author, "converse" in this context means something like "expressing an entailment relation with the same structure for a different set of related concepts."
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