The House of Lords decided that as she did not know her property was being used for that purpose, she could not be guilty of the offence with which she was charged. She was a sub-tenant of a farm who had let it to students but retained a room for her own occasional use. Lord Reid said, ‘[W]henever a section is silent as to the mens rea there is a presumption that . . . we must read in words appropriate to require mens rea.'
Source: P109, How the Law Works, Gary Slapper
What's the grammatical structure here? How does one read IN words?
Does this sentence mean 'we must read words appropriately to require mens rea'? If so, why not just write this? If not, what are the similarities and differences ?