How do we arrive at a working consensus?

How do we arrive at a working consensus?

"The fatal tendency of mankind to leave off thinking about a thing when it is no longer doubtful, is the cause of half their errors" (Mill 50).

Mill often emphasizes that "teachers and learners go to sleep at their post, as soon as there is no enemy in the field" (49). However, even he seems to acknowledge that some kind of consensus, while certainly not final or definitive, must often be arrived at in order for any significant progress to be made. Where does one draw the line as far as when to decide that a valid or useful working consensus is possible?