The Lunar Society evolved through various degrees of organisation over a period of up to fifty years, but was only ever an informal group. No constitution, minutes, publications or membership lists survive from any period, and evidence of its existence and activities is found only in the correspondence and notes of those associated with it. Historians therefore disagree on what qualifies as membership of the Lunar Society, who can be considered to have been members, and even when the society can be said to have existed. Josiah Wedgwood, for example, is described by some commentators as being one of five "principal members" of the society, while others consider that he "cannot be recognized as full member" at all. Dates given for the establishment of the society range from "sometime before 1760" to 1775. Some historians argue that it had ceased to exist by 1791; others that it was still operating as late as 1813.