A “once in a generation” discovery of a centuries-old manuscript by John Locke shows the great English philosopher making his earliest arguments for religious toleration, with the scholar who unearthed it calling the document “the origin and catalyst for momentous and foundational ideas of western liberal democracy”.

Dated to 1667-8, the manuscript titled “Reasons for tolerateing Papists equally with others”, was previously unknown to academia. It had been owned by the descendants of one of Locke’s friends until the 1920s, when it was sold at auction to a book dealer. From there, it went into private collections until it was donated to St John’s College, Annapolis, in the latter half of the 20th century. It lay unstudied in archives until Locke scholar JC Walmsley noticed a reference to it in a 1928 book dealer’s catalogue, and raised an eyebrow: Locke, a hugely influential Enlightenment thinker, was not known to have extended his arguments for religious tolerance to Catholics.

Facebook Twitter Pinterest ‘Reasons for tolerateing Papists equally with others’. Photograph: St John’s College, Annapolis

“This [title] sounded entirely unlikely to me,” Walmsley said. “Locke was known for not extending his toleration to Catholics, and I checked through the online listing of Locke manuscripts to see if it even remotely matched a description of any known Locke manuscript. It did not. This suggested one of two things; that the manuscript described was misattributed to Locke – which happens more often than might be supposed – or there was an unknown Locke manuscript which had some very surprising content.”

Scans showed the attribution was correct, while further research revealed it had previously been unknown to scholars.

“Locke is supposed to have never tolerated Catholics,” said Walmsley. “All his published work suggested that he would never even consider this as a possibility. This manuscript shows him taking an initial position that’s startling for him and for thinkers of his time - next to no one suggested this at this point. It shows him to be much more tolerant in certain respects than was ever previously supposed.”

Locke, who died in 1704, is known for his Two Treatises on Government, which which became a foundational text for modern western democracy. His other hugely influential texts included the Essay Concerning Human Understanding, which provided philosophical grounds for the scientific revolution, and A Letter Concerning Toleration, which influenced James Madison’s thinking on the separation of church and state in his work on the US constitution.

In A Letter Concerning Toleration, Locke argued for tolerance at a time when religious uniformity was enforced by law in the wake of the Reformation. In the newly discovered manuscript, Locke first argues impartially for tolerating Catholics, and secondly against their toleration.

“If Papists can be supposd to be as good subjects as others they may be equally tolerated,” he writes. And: “If all subjects should be equally countenanced, & imployd by the Prince. the Papist[s] have an equall title.”

The work was written before 1689’s A Letter Concerning Toleration and is therefore the earliest outing for ideas that would make an indelible impression on western thought.

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“This manuscript is the origin and catalyst for momentous and foundational ideas of western liberal democracy – which did include Catholics,” said Walmsley, who called the discovery “the culmination of a lifetime’s work”. He has just published an essay in the Historical Journal about the discovery, co-authored with Cambridge lecturer Felix Waldmann, who called the manuscript “a crucial and wholly unexpected part of Locke’s intellectual development”.

JR Milton, general editor of the Clarendon Edition of the Works of John Locke and a professor of the history of philosophy at King’s College London, called the discovery very significant, and “a valuable addition to the corpus of Locke’s writings”.

Joseph Macfarland, dean of St John’s College, said it was “an unexpected pleasure to find that we are in possession of a manuscript by Locke himself on a question so critical to American political life and to liberal democracy generally”.

“I hope that this manuscript will draw further attention to the great debt we owe to Locke’s philosophic and pragmatic thinking,” said Macfarland.