To set down is to write something so that it is on record. Fixed in words, and no longer only in somebody's memory.
It is formal, and rules, principles and conditions are set down in documents, which is where the phrase does most of its work. It splits round an object. There is a plain physical sense of putting something on a surface, and a transport sense, since a bus sets down passengers. Lay down does much the same job for rules and is slightly commoner.
Practised in Phrasal Verbs, A Man Called Ove
First appears in chapter 32 of A Man Called Ove.
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