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(Sir Edward) Coke further noted that legal disputes about such matters as inheritance of goods:
are not to be decided by natural reason but by the artificial reason and judgment of law, which law is an art which requires long study and experience, before that a man can attain to the cognisance of it: that the law was the golden metwand and measure to try the causes of the subjects; and which protected his majesty in safety and peace. . . .
I know that this was written in 1607, but:
How would you determine the purpose of which? It doesn't sound like the modern relative pronoun; "which is an art which" sounds curious. This refers to Why does legal English sometimes repeat the antecedent noun after "which"?.
I think that is optional, but is it truly? Why or why not?
This doesn't duplicate; the linked question doesn't answer question 2.
Source: P13, How the Law Works, Gary Slapper