In 1658 he assisted Baxter to draw up the "Fundamentals of Religion."
Caspar Koolhaes, the heroic minister of Leiden - its first lecturer, too, in divinity - pleaded against a too rigid uniformity, for such an agreement on "fundamentals" as had allowed Reformed, Lutherans and Anabaptists to unite.
Theoretical conceptions were revived by Stahl, who held that acids were the fundamentals of all salts, and the erroneous idea that sulphuric acid was the principle of all acids.
But Sauveur fixed the limiting number of beats for the discord far too low, and again he gave no account of dissonances such as the seventh, where the frequency of the beats between the fundamentals is far beyond the number which is unpleasant.
It is worthy of notice that this intercourse with Cromwell occurred when Baxter was summoned to London to assist in settling "the fundamentals of religion," and made the memorable declaration, in answer to the objection that what he had proposed as fundamental "might be subscribed by a Papist or Socinian," - "So much the better, and so much the fitter it is to be the matter of concord."