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Full Idea
"Safety goggles must be worn in the building" is clear enough, but "dogs must always be carried on the escalator" doesn't require us to head off in search of a dog.
Gist of Idea
Sentences of apparent identical form can have different contextual meanings
Source
Keith Devlin (Goodbye Descartes [1997], Ch. 1)
Book Ref
Devlin,Keith: 'Goodbye Descartes: the end of logic' [Wiley 1997], p.7
A Reaction
A nice illustration of how the requirements of logical form will often take us beyond the strict and literal meaning of a sentence, into context, tone, allusion and subjective aspects.