The history of Freemasonry encompasses the origins, evolution and defining events of the known as Freemasonry. It covers three phases. Firstly, the emergence of organised lodges of operative masons during the Middle Ages, then the admission of lay members as "accepted" (a term reflecting the ceremonial “acception” process that made non stone masons members of an operative lodge) or "speculative" masons, and finally the evolution of purely speculative lodges, and the emergence of Grand Lodges to govern them. The watershed in this process is generally taken to be the formation of the first Grand Lodge in London in 1717. The two difficulties facing historians are the paucity of written material, even down to the 19th century, and the misinformation generated by masons and non-masons alike from the earliest years.