Deontic Dualism

In a recent facebook thread, Josh Dever wrote: 

…permission-creating deontic modals (roughly) don't embed in a lot of environments. So "If you can have a cookie, you can have a brownie" can't be permission-creating for cookies, but can be (conditionally) for brownies. "Probably you can have a cookie" can't be permission-creating, but only permission-reporting…I suspect you can make the embedding facts work out with either (i) a semantic story on which there are two different modals or (ii) a pragmatic story on which permission-creation is a speech-acty consequence of using a solitary modal, and the embeddings stop the speech-actiness. I'm generally inclined to type-(i) explanations, but type-(ii) is kind of tempting here.


To which Richard Stillman responded:

If we hold that there are two different types of deontic modal (as opposed to two types of speech act that can be generated using one type of modal), will we have difficulty capturing the logical validity of certain inferences?
E.g., this seems like a valid application of modus ponens:

  1. You may use a number two pencil (permission granting).
  2. If you may use a number two pencil, then you may also use an eraser.
  3. So, you may also use an eraser. 
Would this be equivocation on the two modal view?

I answered that:

Yes, it would be an equivocation, but dualists have a very good (imho) explanation of why this SEEMS like a valid application of modus ponens even though it is not really one:
(1) is a performative act that makes the proposition that is the actual antecedent of the conditional in (2) true. Therefore, if the act corresponding to (1) is felicitous and (2) is true, then the truth of (3) follows of necessity.

In other words, the argument can be easily mistaken for a valid modus ponens because of the close ontological link between this sort of performatives and the corresponding homophonous descriptive statement: that if (1) is felicitous, then (1’) is true.

1.  You may use a number two pencil (permission granting).

1’. You may use a number two pencil (descriptive).


In this explanation, I have assumed that the consequent of (2) and (3) are descriptive, but the explanations extends easily to the case where they are permission granting.

I also added that if you believe there is something like the logic of permission granting, then the argument would still be logically valid, even if not an actual modus ponens.

Notice that the pattern extends to analogous combinations of performatives and declaratives, like:


  1. The equipment will arrive on time (promise).
  2. If the equipment arrives on time, then it'll be ready for the presentation.
  3. So, the equipment will be ready for the presentation. 

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