Germanic modal auxiliaries

From: Andrew Jarrette
Message: 48962
Date: 2007-06-12

I've always been fascinated about how Gmc *mo:tan (OE mo:tan, OHG muozzan) evolved from "be allowed to" to "be obliged to" (Engl must, German muessen).  I never understood why this change occurred.  According to some dictionaries, the meaning "be obliged to" was already documented in OE times (and I think in OHG times as well), so it looks as though there was some confusion between the ideas of "being allowed" to do something and "being obliged" to do something.  But over the years I reached my own conclusion about how this evolution in meaning occurred.
My idea arose from the fact that the negative of "must" in English ("must not") does not mean "I am not obliged to" but rather means "it is imperative that I do not..." (as opposed to "may not" meaning "I am not allowed to").  This latter meaning is clearly an outcome of the meaning "I am not allowed to" (and apparently "Ich darf nicht" in German can mean both "I am not allowed to" and "it is imperative that I do not").  Thus it seems that "ic ne mo:t ga:n" was reinterpreted from "I am not allowed to go" to "it is imperative that I do not go".  Thence the positive of mo:tan took on a new meaning from this negative, and "ic mo:t ga:n" changed from "I am allowed to go" to "it is imperative that I go" (as opposed to "ic ne mo:t ga:n" = "it is imperative that I do not go").   This is the modern meaning of "I must go" in English.  In German the positive meaning of ich muss gehen was further reinterpreted to mean "I am obliged to go, I have to go", and a new negative evolved, ich muss nicht gehen, with the meaning "I am not obliged to go, I don't have to go" (unlike English "I must not go").  This is my idea of how the meaning of "be obliged to" evolved for German muessen and partially for English must.
I think a similar process happened in the case of German duerfen.  From ih ne darf ge:n meaning "I need not go" evolved a new interpretation as "I am free to not go".  Then ih darf ge:n took on the positive meaning of this new interpretation, "I am free to go", thence "I am allowed to go", and the negative ich darf nicht gehen became the negative of this new positive, "I am not allowed to go".
This explanation of the evolution of the meanings of these West Germanic modal auxiliaries may be self-evident, but I have never seen it expounded anywhere.  Has a similar explanation been published anywhere?  If not, what do the readers of Cybalist think of it?  Does it sound plausible or likely?  Or is there a better explanation for the shift of meaning of muessen and duerfen in German and of must in English?